diff --git a/icu4c/source/common/umutex.cpp b/icu4c/source/common/umutex.cpp
index 8be43bf960c..fe50d866d4f 100644
--- a/icu4c/source/common/umutex.cpp
+++ b/icu4c/source/common/umutex.cpp
@@ -98,6 +98,7 @@ std::mutex *UMutex::getMutex() {
             gListHead = this;
         }
     }
+    U_ASSERT(retPtr != nullptr);
     return retPtr;
 }
 
diff --git a/icu4c/source/common/umutex.h b/icu4c/source/common/umutex.h
index 13e38b49aae..d154ff84339 100755
--- a/icu4c/source/common/umutex.h
+++ b/icu4c/source/common/umutex.h
@@ -240,7 +240,8 @@ private:
     static UMutex *gListHead;
 
     /** Out-of-line function to lazily initialize a UMutex on first use.
-     * Initial fast check is inline, in lock().
+     * Initial fast check is inline, in lock().  The returned value may never
+     * be nullptr.
      */
     std::mutex *getMutex();
 };