diff --git a/DEVELOP.md b/DEVELOP.md
index 807bfc0..df5d824 100644
--- a/DEVELOP.md
+++ b/DEVELOP.md
@@ -89,3 +89,4 @@ MShellObject
 
 [fish shell built in](https://github.com/fish-shell/fish-shell/tree/master/src/builtins)
 [Stroustrop's Rule](https://buttondown.com/hillelwayne/archive/stroustrops-rule/)
+[WebAssembly Type Checking](https://binji.github.io/posts/webassembly-type-checking/)
diff --git a/mshell/Evaluator.go b/mshell/Evaluator.go
index 0c17018..f4ba04c 100644
--- a/mshell/Evaluator.go
+++ b/mshell/Evaluator.go
@@ -1324,9 +1324,16 @@ MainLoop:
 
 				breakDiff := 0
 
+				initialStackSize := len(*stack)
+
 				for loopCount < maxLoops {
 					result := state.Evaluate(quotation.Tokens, stack, loopContext, definitions)
 
+					if len(*stack) != initialStackSize {
+						// If the stack size changed, we have an error.
+						return FailWithMessage(fmt.Sprintf("%d:%d: Stack size changed from %d to %d in loop.\n", t.Line, t.Column, initialStackSize, len(*stack)))
+					}
+
 					if !result.Success {
 						return result
 					}
diff --git a/tests/loop_test.msh b/tests/loop_test.msh
index 20cc179..8e9228a 100644
--- a/tests/loop_test.msh
+++ b/tests/loop_test.msh
@@ -1,7 +1,8 @@
 [
     echo
-    (
-        hello world
-        [ (10 10 =) (break)] if
-    ) loop
-] ;
+]
+(
+    hello append world append
+    [ (10 10 =) (break)] if
+) loop
+;
diff --git a/tests/nested_loop.msh b/tests/nested_loop.msh
index 012425e..46c5c4c 100644
--- a/tests/nested_loop.msh
+++ b/tests/nested_loop.msh
@@ -1,6 +1,6 @@
 0 outer-inc!
 (
-    "Outer: " @outer-inc str wl
+    "Outer: " @outer-inc str + wl
     0 inner-inc!
     (
         " Inner: " w @inner-inc str wl
diff --git a/tests/nested_loop.msh.stdout b/tests/nested_loop.msh.stdout
index 675fdb5..c2dd640 100644
--- a/tests/nested_loop.msh.stdout
+++ b/tests/nested_loop.msh.stdout
@@ -1,19 +1,19 @@
-0
+Outer: 0
  Inner: 0
  Inner: 1
  Inner: 2
  Inner: 3
-1
+Outer: 1
  Inner: 0
  Inner: 1
  Inner: 2
  Inner: 3
-2
+Outer: 2
  Inner: 0
  Inner: 1
  Inner: 2
  Inner: 3
-3
+Outer: 3
  Inner: 0
  Inner: 1
  Inner: 2
diff --git a/tests/while_read.msh b/tests/while_read.msh
index d14dcf8..6c5521f 100644
--- a/tests/while_read.msh
+++ b/tests/while_read.msh
@@ -1,5 +1,5 @@
 1
 (
-    read  [(not) (break)] if
+    read  [(not) (drop break)] if
     linetext! num! "Line " @num str ": " + + @linetext + [echo] append ; @num 1 +
 ) "stdin_for_test.txt" < loop