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
@@ -317,7 +317,7 @@
   } else {
     // CLOSE(UNIT=bad unit) is just a no-op
     Terminator oom{sourceFile, sourceLine};
-    return &New<NoopCloseStatementState>{oom}(sourceFile, sourceLine)
+    return &New<NoopStatementState>{oom}(sourceFile, sourceLine)
                 .release()
                 ->ioStatementState();
   }
@@ -325,11 +325,16 @@
 
 Cookie IONAME(BeginFlush)(
     ExternalUnit unitNumber, const char *sourceFile, int sourceLine) {
-  Terminator terminator{sourceFile, sourceLine};
-  ExternalFileUnit &unit{
-      ExternalFileUnit::LookUpOrCrash(unitNumber, terminator)};
-  return &unit.BeginIoStatement<ExternalMiscIoStatementState>(
-      unit, ExternalMiscIoStatementState::Flush, sourceFile, sourceLine);
+  if (ExternalFileUnit * unit{ExternalFileUnit::LookUp(unitNumber)}) {
+    return &unit->BeginIoStatement<ExternalMiscIoStatementState>(
+        *unit, ExternalMiscIoStatementState::Flush, sourceFile, sourceLine);
+  } else {
+    // FLUSH(UNIT=unknown) is a no-op
+    Terminator oom{sourceFile, sourceLine};
+    return &New<NoopStatementState>{oom}(sourceFile, sourceLine)
+                .release()
+                ->ioStatementState();
+  }
 }
 
 Cookie IONAME(BeginBackspace)(
@@ -880,7 +885,7 @@
     }
     return false;
   }
-  if (io.get_if<NoopCloseStatementState>()) {
+  if (io.get_if<NoopStatementState>()) {
     return true; // don't bother validating STATUS= in a no-op CLOSE
   }
   io.GetIoErrorHandler().Crash(
diff --git a/flang/runtime/io-stmt.h b/flang/runtime/io-stmt.h
--- a/flang/runtime/io-stmt.h
+++ b/flang/runtime/io-stmt.h
@@ -34,7 +34,7 @@
 class InquireIOLengthState;
 class ExternalMiscIoStatementState;
 class CloseStatementState;
-class NoopCloseStatementState;
+class NoopStatementState; // CLOSE or FLUSH on unknown unit
 
 template <Direction, typename CHAR = char>
 class InternalFormattedIoStatementState;
@@ -238,7 +238,7 @@
 private:
   std::variant<std::reference_wrapper<OpenStatementState>,
       std::reference_wrapper<CloseStatementState>,
-      std::reference_wrapper<NoopCloseStatementState>,
+      std::reference_wrapper<NoopStatementState>,
       std::reference_wrapper<
           InternalFormattedIoStatementState<Direction::Output>>,
       std::reference_wrapper<
@@ -616,9 +616,9 @@
   ConnectionState connection_;
 };
 
-class NoopCloseStatementState : public NoUnitIoStatementState {
+class NoopStatementState : public NoUnitIoStatementState {
 public:
-  NoopCloseStatementState(const char *sourceFile, int sourceLine)
+  NoopStatementState(const char *sourceFile, int sourceLine)
       : NoUnitIoStatementState{sourceFile, sourceLine, *this} {}
   void set_status(CloseStatus) {} // discards
 };