diff --git a/src/Lean/Compiler/IR/ToIR.lean b/src/Lean/Compiler/IR/ToIR.lean
index fc5b6cc451..d67fdcb2e7 100644
--- a/src/Lean/Compiler/IR/ToIR.lean
+++ b/src/Lean/Compiler/IR/ToIR.lean
@@ -110,7 +110,8 @@ def lowerType (e : Lean.Expr) : M IRType := do
       else
         return .object
   | .app f _ =>
-    if let .const name _ := f.headBeta then
+    -- All mono types are in headBeta form.
+    if let .const name _ := f then
       if let some scalarType ← lowerEnumToScalarType name then
         return scalarType
       else