Index: flang/lib/Parser/preprocessor.cpp
===================================================================
--- flang/lib/Parser/preprocessor.cpp
+++ flang/lib/Parser/preprocessor.cpp
@@ -169,8 +169,9 @@
           replacement_.TokenAt(prev - 1)[0] ==
               '#') { // stringify argument without macro replacement
         std::size_t resultSize{result.SizeInTokens()};
-        while (resultSize > 0 && result.TokenAt(resultSize - 1).empty()) {
+        while (resultSize > 0 && result.TokenAt(resultSize - 1).IsBlank()) {
           result.pop_back();
+          --resultSize;
         }
         CHECK(resultSize > 0 &&
             result.TokenAt(resultSize - 1) == replacement_.TokenAt(prev - 1));
Index: flang/test/Preprocessing/pp045.F
===================================================================
--- /dev/null
+++ flang/test/Preprocessing/pp045.F
@@ -0,0 +1,9 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: n = "foobar" // "baz"
+* # stringizing works in FLM
+#define STR(x) # x
+#define MAC(a,b) STR(a ## b)
+      program main
+      character(6) n
+      n = MAC(foo, bar) // STR( baz )
+      end
Index: flang/test/Preprocessing/pp131.F90
===================================================================
--- /dev/null
+++ flang/test/Preprocessing/pp131.F90
@@ -0,0 +1,9 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: n = "foobar" // "baz"
+! # stringizing works in FLM
+#define STR(x) # x
+#define MAC(a,b) STR(a ## b)
+program main
+  character(6) n
+  n = MAC(foo, bar) // STR( baz )
+end