diff --git a/icu4c/source/common/ucnv_ext.c b/icu4c/source/common/ucnv_ext.c
index 8e218616fec..e358a8b2fe9 100644
--- a/icu4c/source/common/ucnv_ext.c
+++ b/icu4c/source/common/ucnv_ext.c
@@ -663,7 +663,7 @@ ucnv_extMatchFromU(const int32_t *cx,
 
     /* return result */
     if(matchValue==UCNV_EXT_FROM_U_SUBCHAR1) {
-        return 1;
+        return 1; /* assert matchLength==2 */
     }
 
     *pMatchValue=UCNV_EXT_FROM_U_MASK_ROUNDTRIP(matchValue);
diff --git a/icu4c/source/common/ucnv_ext.h b/icu4c/source/common/ucnv_ext.h
index 29a683263f0..361ba73948c 100644
--- a/icu4c/source/common/ucnv_ext.h
+++ b/icu4c/source/common/ucnv_ext.h
@@ -269,6 +269,12 @@
  *   (i.e., "no mapping" with preference for <subchar1> rather than <subchar>),
  *   and all other items are unique non-zero results.
  *
+ *   The default value of a fromUTableValues[] section that is referenced
+ *   _directly_ from a fromUStage3b[] item may also be UCNV_EXT_FROM_U_SUBCHAR1,
+ *   but this value must not occur anywhere else in fromUTableValues[]
+ *   because "no mapping" is always a property of a single code point,
+ *   never of multiple.
+ *
  *
  * char fromUBytes[];
  *
diff --git a/icu4c/source/common/ucnvmbcs.c b/icu4c/source/common/ucnvmbcs.c
index 6d8c8b836d1..d0b805b4c2c 100644
--- a/icu4c/source/common/ucnvmbcs.c
+++ b/icu4c/source/common/ucnvmbcs.c
@@ -3026,7 +3026,9 @@ getTrail:
                 cnv->fromUnicodeStatus=prevLength; /* save the old state */
                 value=MBCS_VALUE_2_FROM_STAGE_2(bytes, stage2Entry, c);
                 if(value<=0xff) {
-                    if(prevLength<=1) {
+                    if(value==0 && MBCS_FROM_U_IS_ROUNDTRIP(stage2Entry, c)==0) {
+                        /* no mapping, leave value==0 */
+                    } else if(prevLength<=1) {
                         length=1;
                     } else {
                         /* change from double-byte mode to single-byte */