diff -ur icu.org/source/tools/genrb/rbutil.c icu/source/tools/genrb/rbutil.c
--- icu.org/source/tools/genrb/rbutil.c	2020-10-28 22:21:12.000000000 +0100
+++ icu/source/tools/genrb/rbutil.c	2020-11-16 19:50:44.005119253 +0100
@@ -30,7 +30,12 @@
 get_dirname(char *dirname,
             const char *filename)
 {
-  const char *lastSlash = uprv_strrchr(filename, U_FILE_SEP_CHAR) + 1;
+  const char *lastSlash = uprv_strrchr(filename, U_FILE_SEP_CHAR);
+  if(lastSlash == NULL) {
+    lastSlash = filename;
+  } else {
+     ++lastSlash;
+  }
 
   if(lastSlash>filename) {
     uprv_strncpy(dirname, filename, (lastSlash - filename));
@@ -46,7 +51,12 @@
              const char *filename)
 {
   /* strip off any leading directory portions */
-  const char *lastSlash = uprv_strrchr(filename, U_FILE_SEP_CHAR) + 1;
+  const char *lastSlash = uprv_strrchr(filename, U_FILE_SEP_CHAR);
+  if(lastSlash == NULL) {
+    lastSlash = filename;
+  } else {
+     ++lastSlash;
+  }
   char *lastDot;
 
   if(lastSlash>filename) {