diff --git a/icu4c/source/common/punycode.cpp b/icu4c/source/common/punycode.cpp
index aa02298c5e6d..1868a07a856a 100644
--- a/icu4c/source/common/punycode.cpp
+++ b/icu4c/source/common/punycode.cpp
@@ -193,7 +193,7 @@ u_strToPunycode(const char16_t *src, int32_t srcLength,
         return 0;
     }
 
-    if(src==nullptr || srcLength<-1 || (dest==nullptr && destCapacity!=0)) {
+    if(src==nullptr || srcLength<-1 || destCapacity<0 || (dest==nullptr && destCapacity!=0)) {
         *pErrorCode=U_ILLEGAL_ARGUMENT_ERROR;
         return 0;
     }