diff --git a/builtins/2and3/cmath.pyi b/builtins/2and3/cmath.pyi
index 9bca157b7..9fc13c0a9 100644
--- a/builtins/2and3/cmath.pyi
+++ b/builtins/2and3/cmath.pyi
@@ -28,3 +28,6 @@ def sinh(x:_C) -> complex: ...
 def sqrt(x:_C) -> complex: ...
 def tan(x:_C) -> complex: ...
 def tanh(x:_C) -> complex: ...
+
+if sys.version_info[0] >= 3:
+    def isfinite(z:_C) -> bool: ...