diff --git a/xfa/fgas/crt/cfgas_formatstring.cpp b/xfa/fgas/crt/cfgas_formatstring.cpp
index d07f14f..4f766c7 100644
--- a/xfa/fgas/crt/cfgas_formatstring.cpp
+++ b/xfa/fgas/crt/cfgas_formatstring.cpp
@@ -1855,8 +1855,8 @@
 bool CFGAS_FormatString::FormatStrNum(WideStringView wsInputNum,
                                       const WideString& wsPattern,
                                       WideString* wsOutput) const {
-  if (wsInputNum.IsEmpty() || wsPattern.IsEmpty())
-    return false;
+  ASSERT(!wsInputNum.IsEmpty());
+  ASSERT(!wsPattern.IsEmpty());
 
   int32_t dot_index_f = -1;
   uint32_t dwNumStyle = 0;
@@ -2019,6 +2019,9 @@
 
           ccf -= 2;
           bAddNeg = true;
+        } else {
+          wsOutput->InsertAtFront('r');
+          ccf--;
         }
         break;
       case 'R':
@@ -2026,6 +2029,9 @@
           *wsOutput = bNeg ? L"CR" : L"  " + *wsOutput;
           ccf -= 2;
           bAddNeg = true;
+        } else {
+          wsOutput->InsertAtFront('R');
+          ccf--;
         }
         break;
       case 'b':
@@ -2035,6 +2041,9 @@
 
           ccf -= 2;
           bAddNeg = true;
+        } else {
+          wsOutput->InsertAtFront('b');
+          ccf--;
         }
         break;
       case 'B':
@@ -2042,6 +2051,9 @@
           *wsOutput = bNeg ? L"DB" : L"  " + *wsOutput;
           ccf -= 2;
           bAddNeg = true;
+        } else {
+          wsOutput->InsertAtFront('B');
+          ccf--;
         }
         break;
       case '%': {
@@ -2234,6 +2246,7 @@
                                    WideString* wsOutput) const {
   if (wsSrcNum.IsEmpty() || wsPattern.IsEmpty())
     return false;
+
   return FormatStrNum(wsSrcNum.AsStringView(), wsPattern, wsOutput);
 }
 
diff --git a/xfa/fgas/crt/cfgas_formatstring_unittest.cpp b/xfa/fgas/crt/cfgas_formatstring_unittest.cpp
index 3a8073c..e961265 100644
--- a/xfa/fgas/crt/cfgas_formatstring_unittest.cpp
+++ b/xfa/fgas/crt/cfgas_formatstring_unittest.cpp
@@ -509,6 +509,11 @@
       // {L"en", L"-123.5", L"zzz.z)", L"123.5)"},
       {L"en", L"123.5", L"zzz.z)", L"123.5 "},
       {L"en", L"123.5", L"zzz.z(", L"123.5 "},
+      // https://crbug.com/pdfium/1233
+      {L"en", L"1", L"r9", L"r1"},
+      {L"en", L"1", L"R9", L"R1"},
+      {L"en", L"1", L"b9", L"b1"},
+      {L"en", L"1", L"B9", L"B1"},
   };
 
   for (size_t i = 0; i < FX_ArraySize(tests); ++i) {