diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index ec90fb3e302..5b792b08e28 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -6491,7 +6491,7 @@ static std::vector<ValueFlow::Value> getContainerSizeFromConstructorArgs(const s
             if (args.size() == 1 && args[0]->tokType() == Token::Type::eString)
                 return {makeContainerSizeValue(Token::getStrLength(args[0]), known)};
             if (args.size() == 1 && args[0]->variable() && args[0]->variable()->isArray() &&
-                args[0]->variable()->isConst() && args[0]->variable()->dimensions().size() == 1)
+                args[0]->variable()->isConst() && args[0]->variable()->dimensions().size() == 1 && args[0]->variable()->dimensions()[0].known)
                 return {makeContainerSizeValue(args[0]->variable()->dimensions()[0].num, known)};
             if (args.size() == 2 && astIsIntegral(args[1], false)) // { char*, count }
                 return {makeContainerSizeValue(args[1], known)};
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index 79ad6e45b30..6eb807bdd11 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -7406,6 +7406,12 @@ class TestValueFlow : public TestFixture {
                "    return a[0];\n"
                "}";
         ASSERT(!isKnownContainerSizeValue(tokenValues(code, "a ["), 6).empty());
+
+        code = "void f(const char a[]) {\n" // #14518
+               "    std::string s(a);\n"
+               "    if (s.empty()) {}\n"
+               "}";
+        ASSERT(!isKnownContainerSizeValue(tokenValues(code, "s ."), 0).empty());
     }
 
     void valueFlowContainerElement()