diff --git a/src/Convert/Cast.hs b/src/Convert/Cast.hs
index fbe01b1..b793516 100644
--- a/src/Convert/Cast.hs
+++ b/src/Convert/Cast.hs
@@ -181,7 +181,8 @@ castFnName size signed =
             Number n ->
                 case numberToInteger n of
                     Just v -> show v
-                    _ -> shortHash size
+                    _ -> error $ "size cast width " ++ show n
+                            ++ " is not an integer"
             _ -> shortHash size
         suffix = if signed then "_signed" else ""
 
diff --git a/test/error/size_cast_x.sv b/test/error/size_cast_x.sv
new file mode 100644
index 0000000..da144ae
--- /dev/null
+++ b/test/error/size_cast_x.sv
@@ -0,0 +1,4 @@
+// pattern: size cast width 1'bx is not an integer
+module top;
+    initial $display((1'bx)'(2));
+endmodule