diff --git a/src/constraint.rs b/src/constraint.rs
index 35f61f9e4..0fd053888 100644
--- a/src/constraint.rs
+++ b/src/constraint.rs
@@ -21,8 +21,8 @@ use std::{fmt::Debug, iter::once, mem::swap};
 pub enum ImpossibleConstraint {
     ArityMismatch {
         atom: Atom<Symbol>,
+        // The expected arity for this atom
         expected: usize,
-        actual: usize,
     },
     FunctionMismatch {
         expected_output: ArcSort,
@@ -65,14 +65,10 @@ impl ConstraintError<AtomTerm, ArcSort> {
             ConstraintError::ImpossibleCaseIdentified(ImpossibleConstraint::ArityMismatch {
                 atom,
                 expected,
-                actual,
-            }) => {
-                assert_eq!(*actual, atom.args.len() - 1);
-                TypeError::Arity {
-                    expr: atom.to_expr(),
-                    expected: *expected,
-                }
-            }
+            }) => TypeError::Arity {
+                expr: atom.to_expr(),
+                expected: *expected - 1,
+            },
             ConstraintError::ImpossibleCaseIdentified(ImpossibleConstraint::FunctionMismatch {
                 expected_output,
                 expected_input,
@@ -635,8 +631,7 @@ fn get_atom_application_constraints(
                         head: *head,
                         args: args.to_vec(),
                     },
-                    expected: typ.input.len(),
-                    actual: args.len() - 1,
+                    expected: typ.input.len() + 1,
                 },
             ));
         } else {
@@ -735,7 +730,6 @@ impl TypeConstraint for SimpleTypeConstraint {
                         args: arguments.to_vec(),
                     },
                     expected: self.sorts.len(),
-                    actual: arguments.len(),
                 },
             )]
         } else {
@@ -815,7 +809,6 @@ impl TypeConstraint for AllEqualTypeConstraint {
                             args: arguments.to_vec(),
                         },
                         expected: exact_length,
-                        actual: arguments.len(),
                     },
                 )]
             }
diff --git a/src/sort/fn.rs b/src/sort/fn.rs
index 83e683f1b..652a11b77 100644
--- a/src/sort/fn.rs
+++ b/src/sort/fn.rs
@@ -258,8 +258,7 @@ impl TypeConstraint for FunctionCTorTypeConstraint {
                         head: self.name,
                         args: arguments.to_vec(),
                     },
-                    expected: 1,
-                    actual: 0,
+                    expected: 2,
                 },
             )];
         }
@@ -284,7 +283,6 @@ impl TypeConstraint for FunctionCTorTypeConstraint {
                                 args: arguments.to_vec(),
                             },
                             expected: self.function.inputs.len() + func_type.input.len() + 1,
-                            actual: arguments.len() - 1,
                         },
                     )];
                 }
diff --git a/tests/fail-typecheck/arity-mismatch.egg b/tests/fail-typecheck/arity-mismatch.egg
new file mode 100644
index 000000000..1a9372c5c
--- /dev/null
+++ b/tests/fail-typecheck/arity-mismatch.egg
@@ -0,0 +1 @@
+(+ 1 2 3)
diff --git a/tests/fail-typecheck/set-a-primitive.egg b/tests/fail-typecheck/set-a-primitive.egg
new file mode 100644
index 000000000..cd564f549
--- /dev/null
+++ b/tests/fail-typecheck/set-a-primitive.egg
@@ -0,0 +1 @@
+(set (+ 1 2) 3)