diff --git a/changelog.txt b/changelog.txt
index 0416945e71..4a3d986028 100644
--- a/changelog.txt
+++ b/changelog.txt
@@ -111,7 +111,7 @@ Version 0.91.0:
 - fbc crashed after showing the error message when using -vec 1|2 without -fpu sse
 - ASM backend: Sgn(LongInt) returned wrong values
 - C backend: Extern "Windows-MS" is now supported, including DLL exports (stdcall procedures without @N suffix)
-
+- #709: format() adds erroneous thousands delimiter, was affecting all number strings that were multiple of 3. 
 
 Version 0.90.1:
 
diff --git a/src/rtlib/str_format.c b/src/rtlib/str_format.c
index 2fbc9b00b2..0a0f3d198f 100644
--- a/src/rtlib/str_format.c
+++ b/src/rtlib/str_format.c
@@ -453,7 +453,7 @@ int fb_hProcessMask
                         DBG_ASSERT( IndexFix!=LenFix );
                         pszAdd = FixPart + IndexFix;
                         if( pInfo->has_thousand_sep ) {
-                            int remaining = LenFix - IndexFix;
+                            int remaining = LenFix - IndexFix - 1;
                             if( (remaining % 3)==0 ) {
                                 if( did_thousandsep ) {
                                     did_thousandsep = FALSE;