diff --git a/compat/win32/syslog.c b/compat/win32/syslog.c
index 161978d720..1f8d8934cc 100644
--- a/compat/win32/syslog.c
+++ b/compat/win32/syslog.c
@@ -43,6 +43,7 @@ void syslog(int priority, const char *fmt, ...)
 	va_end(ap);
 
 	while ((pos = strstr(str, "%1")) != NULL) {
+		size_t offset = pos - str;
 		char *oldstr = str;
 		str = realloc(str, st_add(++str_len, 1));
 		if (!str) {
@@ -50,6 +51,7 @@ void syslog(int priority, const char *fmt, ...)
 			warning_errno("realloc failed");
 			return;
 		}
+		pos = str + offset;
 		memmove(pos + 2, pos + 1, strlen(pos));
 		pos[1] = ' ';
 	}