diff --git a/src/PremadeModels.jl b/src/PremadeModels.jl
index 13ca65a3..b6f9f8e3 100644
--- a/src/PremadeModels.jl
+++ b/src/PremadeModels.jl
@@ -48,8 +48,8 @@ seir_model = @stock_and_flow begin
         tlatent
         trecovery
         δ
-        c 
-        
+        c
+
 
         :dynamic_variables
         v_prevalence = NI / NS
@@ -61,14 +61,14 @@ seir_model = @stock_and_flow begin
 
         v_inf = E / tlatent
 
-        v_rec = I / trecovery 
+        v_rec = I / trecovery
 
 
-        v_deathS = δ * S       
+        v_deathS = δ * S
         v_deathE = δ * E
         v_deathI = δ * I
         v_deathR = δ * R
-        
+
 
         :flows
         CLOUD => f_birth(v_birth) => S
@@ -92,7 +92,7 @@ sis_model = @stock_and_flow begin
     S
     I
 
-    
+
     :parameters
     μ
     β
@@ -100,7 +100,7 @@ sis_model = @stock_and_flow begin
     δ
     c
 
-      
+
     :dynamic_variables
     v_deathsX = δ * S
     v_births = μ * N
@@ -126,7 +126,7 @@ sis_model = @stock_and_flow begin
     N = [S, I]
     NI = [I]
     NS = [S, I]
-  
+
 end
 
 
@@ -135,35 +135,35 @@ sir_model = @stock_and_flow begin
     S
     I
     R
-    
+
     :parameters
     c
     β
     rRec
-    
+
     :dynamic_variables
     v_prevalence = NI / NS
     v_meanInfectiousContactsPerS = c * v_prevalence
     v_perSIncidenceRate = β * v_meanInfectiousContactsPerS
     v_newInfections = S * v_perSIncidenceRate
     v_newRecovery = I * rRec
-    
+
     :flows
     S => f_inf(v_newInfections) => I
     I => f_rec(v_newRecovery) => R
-    
+
     :sums
     N = [S, I, R]
     NI = [I]
     NS = [S,I,R]
-    
-    
+
+
 end
 
 
 
 svi_model = @stock_and_flow begin
-    
+
     :stocks
     S
     V
@@ -177,9 +177,9 @@ svi_model = @stock_and_flow begin
     β
 
     :dynamic_variables
-    v_vacc = S * rvaccine 
+    v_vacc = S * rvaccine
     v_deathV = δ * V
-    
+
     v_prevalence = NI / NS
     v_meanInfectiousContactsPerS = c * v_prevalence
     v_perSIncidenceRate = β * v_meanInfectiousContactsPerS
@@ -190,9 +190,9 @@ svi_model = @stock_and_flow begin
     :flows
     S => f_vacc(v_vacc) => V
     V => f_deathV(v_deathV) => CLOUD
-    V => f_infV(v_perSIncidenceVaccinated) => I 
+    V => f_infV(v_perSIncidenceVaccinated) => I
+
 
-    
     :sums
     N = [S, V, I]
     NI = [I]
diff --git a/src/Syntax.jl b/src/Syntax.jl
index ec5aa14f..457d59ee 100644
--- a/src/Syntax.jl
+++ b/src/Syntax.jl
@@ -134,7 +134,7 @@ Compiles stock and flow syntax of the line-based block form
     symbol_r => flow_name_1(dyvar_k) => symbol_q
     symbol_z => flow_name_2(dyvar_g * param_v) => symbol_p
     ☁       => flow_name_3(symbol_c + dyvar_b) => symbol_r
-    symbol_j => flow_name_4(param_l + symbol_m) => TODO
+    symbol_j => flow_name_4(param_l + symbol_m) => CLOUD
     ...
     symbol_y => flow_name_n(dyvar_f) => ☁
 ```
@@ -142,15 +142,18 @@ into a StockAndFlowF data type for use with the StockFlow.jl modelling system.
 """
 macro stock_and_flow(block)
     Base.remove_linenums!(block)
-    syntax_lines = parse_stock_and_flow_syntax(block.args)
-    saff_args = stock_and_flow_syntax_to_arguments(syntax_lines)
-    return StockAndFlowF(
-        saff_args.stocks,
-        saff_args.params,
-        map(kv -> kv.first => get(kv.second), saff_args.dyvars),
-        saff_args.flows,
-        saff_args.sums,
-    )
+    block_args = block.args
+    return quote
+      local syntax_lines = parse_stock_and_flow_syntax($block_args)
+      local saff_args = stock_and_flow_syntax_to_arguments(syntax_lines)
+      StockAndFlowF(
+          saff_args.stocks,
+          saff_args.params,
+          map(kv -> kv.first => get(kv.second), saff_args.dyvars),
+          saff_args.flows,
+          saff_args.sums,
+      )
+    end
 end
 
 """
@@ -378,7 +381,7 @@ and the flow equations's definition as an expression.
 ### Input
 - `flow_definition` -- A flow definition of the form
                        `SYMBOL => flow_name(flow_equation) => SYMBOL`,
-                       where SYMBOL can be an arbitrary name or special cases of ☁ or TODO,
+                       where SYMBOL can be an arbitrary name or special cases of ☁ or CLOUD,
                        which corresponds to a flow from nowhere.
 
 ### Output
@@ -387,15 +390,15 @@ may be :F_NONE for a flow from nowhere) and the flow equation as a julia express
 
 ### Examples
 ```julia-repl
-julia> Syntax.parse_flow_io(:(TODO => birthRate(a * b * c) => S))
+julia> Syntax.parse_flow_io(:(CLOUD => birthRate(a * b * c) => S))
 (:F_NONE, :(birthRate(a * b * c)), :S)
 ```
 """
 function parse_flow(flow_definition::Expr)
     @match flow_definition begin
-        :(TODO => $flow => $stock_out) || :(☁ => $flow => $stock_out) =>
+        :(CLOUD => $flow => $stock_out) || :(☁ => $flow => $stock_out) =>
             (:F_NONE, flow, stock_out)
-        :($stock_in => $flow => TODO) || :($stock_in => $flow => ☁) =>
+        :($stock_in => $flow => CLOUD) || :($stock_in => $flow => ☁) =>
             (stock_in, flow, :F_NONE)
         :($stock_in => $flow => $stock_out) => (stock_in, flow, stock_out)
         Expr(en, _, _, _) || Expr(en, _, _) =>
@@ -470,6 +473,17 @@ function assemble_stock_definitions(
     flows::Vector{Tuple{Symbol,Expr,Symbol}},
     sum_variables::Vector{Tuple{Symbol,Vector{Symbol}}},
 )
+    # Check that all of the stocks involved in flow definitions exist
+    stock_set = Set(stocks)
+    push!(stock_set, :F_NONE) # for error handling step: any 'clouds' should be F_NONE by now.
+    for (start_object, flow, end_object) in flows
+        if !(start_object in stock_set)
+            error(String(start_object) * " is not a known stock.")
+        elseif !(end_object in stock_set)
+            error(String(end_object) * " is not a known stock.")
+        end
+    end
+
     formatted_stocks = []
     for stock in stocks
         input_arrows::Vector{Symbol} = []
@@ -974,7 +988,7 @@ end
 
 Create foot (StockAndFlow0) using format A => B, where A is a stock and B is a sum variable.  Use () to represent no stock or sum variable.
 To have multiple stocks or sum variables, chain together multiple pairs with commas.  Repeated occurences of the same symbol will be treated as the same stock or sum variable.
-You cannot create distinct stocks or sum variables with the same name using this format. 
+You cannot create distinct stocks or sum variables with the same name using this format.
 
 ```julia
 standard_foot = @foot A => N
@@ -989,8 +1003,8 @@ multiple_links = @foot A => B, A => B # will have two links from A to B.
 function create_foot(block::Expr)
     @match block.head begin
 
-        :tuple => begin 
-            if isempty(block.args) # case for create_foot(:()) 
+        :tuple => begin
+            if isempty(block.args) # case for create_foot(:())
                 error("Cannot create foot with no arguments.")
             end
             foot_s = Vector{Symbol}()
@@ -1016,7 +1030,7 @@ Takes as argument an expression of the form A => B and returns a tuple in a form
 Return type is Tuple{Union{Tuple{}, Symbol}, Union{Tuple{}, Symbol}, Union{Tuple{}, Pair{Symbol, Symbol}}}.  The empty tuple represents no stocks, no flows, or no links.
 
 """
-function match_foot_format(footblock::Expr) 
+function match_foot_format(footblock::Expr)
     @match footblock begin
         :(()              => ())              => ((), (), ())
         :($(s :: Symbol)  => ())              => (s, (), ())
diff --git a/test/Syntax.jl b/test/Syntax.jl
index 36df5324..34f12526 100644
--- a/test/Syntax.jl
+++ b/test/Syntax.jl
@@ -253,16 +253,23 @@ end
 
 end
 @testset "recursive definitions should be disallowed" begin
-    expr = quote
+    @test_throws Exception @stock_and_flow begin
         :dynamic_variables
         v = v + v
     end
-    # When used as a macro -- @stock_and_flow -- this exception is thrown
-    # at a point that @test_throws cannot capture it.
-    @test_throws Exception stock_and_flow(expr)
 end
 
+@testset "the existence of references is checked" begin
+    @test_throws Exception @stock_and_flow begin
+        :stocks
+        A
+        B
+        C
 
+        :flows
+        a => f(A * B) => B
+    end
+end
 
 @testset "foot syntax can create all types of feet" begin
     @test (@foot A => B) == foot(:A, :B, :A => :B)
@@ -270,7 +277,7 @@ end
     @test (@foot () => Q) == foot((), :Q, ())
     @test (@foot () => ()) == foot((),(),())
 
-    @test (@foot =>((), SV)) == foot((),:SV,()) 
+    @test (@foot =>((), SV)) == foot((),:SV,())
     @test (@foot A11 => B22) == foot(:A11, :B22, :A11 => :B22)
 
     @test (@foot () => B, A => ()) == foot(:A, :B, ())
@@ -297,7 +304,7 @@ end
 @testset "feet syntax can create feet" begin
 
     @test (@feet begin
-        
+
         A => B
         C => D
         () => ()
@@ -336,6 +343,3 @@ end
     @test_throws Exception @eval @feet begin A => B; =>(D,E,F) end
     @test_throws Exception @eval @feet begin A => B; 1 => 2; end
 end
-
-
-
diff --git a/test/SystemStructure.jl b/test/SystemStructure.jl
index 6b448205..79e08abc 100644
--- a/test/SystemStructure.jl
+++ b/test/SystemStructure.jl
@@ -4,61 +4,61 @@ using StockFlow.Syntax
 
 empty = @stock_and_flow begin end
 
-p = @stock_and_flow begin 
+p = @stock_and_flow begin
     :stocks
     S
     I
-    
+
     :dynamic_variables
     v = S + I
-    
+
     :parameters
     p1
     p2
-    
+
     :flows
     S => f1(v) => CLOUD
-    
+
     :sums
     N = [S]
     NI = [I]
 end
 
-p_prefixed = @stock_and_flow begin 
+p_prefixed = @stock_and_flow begin
     :stocks
     prefS
     prefI
-    
+
     :dynamic_variables
     prefv = prefS + prefI
-    
+
     :parameters
     prefp1
     prefp2
-    
+
     :flows
     prefS => preff1(prefv) => CLOUD
-    
+
     :sums
     prefN = [prefS]
     prefNI = [prefI]
 end
 
-p_suffixed = @stock_and_flow begin 
+p_suffixed = @stock_and_flow begin
     :stocks
     Ssuf
     Isuf
-    
+
     :dynamic_variables
     vsuf = Ssuf + Isuf
-    
+
     :parameters
     p1suf
     p2suf
-    
+
     :flows
     Ssuf => f1suf(vsuf) => CLOUD
-    
+
     :sums
     Nsuf = [Ssuf]
     NIsuf = [Isuf]
@@ -75,11 +75,9 @@ footA_suf = @foot Ssuf => Nsuf
     @test empty == add_suffix!(deepcopy(empty), "AAA") == add_prefix!(deepcopy(empty), "BBB")
     @test add_prefix!(deepcopy(p), "pref") == p_prefixed
     @test add_suffix!(deepcopy(p), "suf") == p_suffixed
-    
+
     @test empty_foot == add_suffix!(deepcopy(empty_foot), "CCC") == add_prefix!(deepcopy(empty_foot), "DDD")
     @test add_prefix!(deepcopy(footA), "pref") == footA_pref
     @test add_suffix!(deepcopy(footA), "suf") == footA_suf
 
 end
-
-