diff --git a/flang/runtime/io-api.cpp b/flang/runtime/io-api.cpp
--- a/flang/runtime/io-api.cpp
+++ b/flang/runtime/io-api.cpp
@@ -1280,6 +1280,9 @@
 static enum Iostat CheckUnitNumberInRangeImpl(INT unit, bool handleError,
     char *ioMsg, std::size_t ioMsgLength, const char *sourceFile,
     int sourceLine) {
+  static_assert(sizeof(INT) >= sizeof(ExternalUnit),
+      "only intended to be used when the INT to ExternalUnit conversion is "
+      "narrowing");
   if (unit != static_cast<ExternalUnit>(unit)) {
     Terminator oom{sourceFile, sourceLine};
     IoErrorHandler errorHandler{oom};