diff --git a/icu4c/source/common/unorm.cpp b/icu4c/source/common/unorm.cpp
index 4cbc317c731..d37f05e5b8c 100644
--- a/icu4c/source/common/unorm.cpp
+++ b/icu4c/source/common/unorm.cpp
@@ -2220,8 +2220,9 @@ unorm_normalize(const UChar *src, int32_t srcLength,
     }
 
     /* check for overlapping src and destination */
-    if( (src>=dest && src<(dest+destCapacity)) ||
-        (srcLength>0 && dest>=src && dest<(src+srcLength))
+    if( dest!=NULL &&
+        ((src>=dest && src<(dest+destCapacity)) ||
+         (srcLength>0 && dest>=src && dest<(src+srcLength)))
     ) {
         *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
         return 0;
diff --git a/icu4c/source/common/ustring.c b/icu4c/source/common/ustring.c
index 350e8c4991f..c65a01ba812 100644
--- a/icu4c/source/common/ustring.c
+++ b/icu4c/source/common/ustring.c
@@ -603,8 +603,9 @@ u_strCaseMap(UChar *dest, int32_t destCapacity,
     }
 
     /* check for overlapping source and destination */
-    if( (src>=dest && src<(dest+destCapacity)) ||
-        (dest>=src && dest<(src+srcLength))
+    if( dest!=NULL &&
+        ((src>=dest && src<(dest+destCapacity)) ||
+         (dest>=src && dest<(src+srcLength)))
     ) {
         /* overlap: provide a temporary destination buffer and later copy the result */
         if(destCapacity<=(sizeof(buffer)/U_SIZEOF_UCHAR)) {