diff --git a/src/validation/__tests__/VariablesAreInputTypesRule-test.ts b/src/validation/__tests__/VariablesAreInputTypesRule-test.ts
index 5a19fca650..9339aaa33c 100644
--- a/src/validation/__tests__/VariablesAreInputTypesRule-test.ts
+++ b/src/validation/__tests__/VariablesAreInputTypesRule-test.ts
@@ -21,6 +21,23 @@ describe('Validate: Variables are input types', () => {
     `);
   });
 
+  it('unknown types are invalid', () => {
+    expectErrors(`
+      query Foo($a: Unknown, $b: [[Unknown!]]!) {
+        field(a: $a, b: $b)
+      }
+    `).to.deep.equal([
+      {
+        locations: [{ line: 2, column: 21 }],
+        message: 'Variable "$a" references unknown type "Unknown".',
+      },
+      {
+        locations: [{ line: 2, column: 34 }],
+        message: 'Variable "$b" references unknown type "[[Unknown!]]!".',
+      },
+    ]);
+  });
+
   it('output types are invalid', () => {
     expectErrors(`
       query Foo($a: Dog, $b: [[CatOrDog!]]!, $c: Pet) {
diff --git a/src/validation/rules/VariablesAreInputTypesRule.ts b/src/validation/rules/VariablesAreInputTypesRule.ts
index 0dc9daa250..38154e2fb5 100644
--- a/src/validation/rules/VariablesAreInputTypesRule.ts
+++ b/src/validation/rules/VariablesAreInputTypesRule.ts
@@ -22,14 +22,15 @@ export function VariablesAreInputTypesRule(
   return {
     VariableDefinition(node: VariableDefinitionNode) {
       const type = typeFromAST(context.getSchema(), node.type);
-
-      if (type && !isInputType(type)) {
+      if (!isInputType(type)) {
         const variableName = node.variable.name.value;
-        const typeName = print(node.type);
+        const typeReference = print(node.type);
 
         context.reportError(
           new GraphQLError(
-            `Variable "$${variableName}" cannot be non-input type "${typeName}".`,
+            type === undefined
+              ? `Variable "$${variableName}" references unknown type "${typeReference}".`
+              : `Variable "$${variableName}" cannot be non-input type "${typeReference}".`,
             node.type,
           ),
         );