diff --git a/lib/valueflow.cpp b/lib/valueflow.cpp
index 8885fc65d45..fac53ecb3f2 100644
--- a/lib/valueflow.cpp
+++ b/lib/valueflow.cpp
@@ -7094,6 +7094,20 @@ static void valueFlowContainerSize(const TokenList& tokenlist,
         }
     }
 
+    auto forwardMinimumContainerSize = [&](MathLib::bigint size, Token* opTok, const Token* exprTok) -> void {
+        if (size == 0)
+            return;
+
+        ValueFlow::Value value(size - 1);
+        value.valueType = ValueFlow::Value::ValueType::CONTAINER_SIZE;
+        value.bound = ValueFlow::Value::Bound::Upper;
+        value.setImpossible();
+        Token* next = nextAfterAstRightmostLeaf(opTok);
+        if (!next)
+            next = opTok->next();
+        valueFlowForward(next, exprTok, std::move(value), tokenlist, errorLogger, settings);
+    };
+
     // after assignment
     for (const Scope *functionScope : symboldatabase.functionScopes) {
         for (auto* tok = const_cast<Token*>(functionScope->bodyStart); tok != functionScope->bodyEnd; tok = tok->next()) {
@@ -7161,20 +7175,22 @@ static void valueFlowContainerSize(const TokenList& tokenlist,
                     value.setImpossible();
                     valueFlowForward(tok->linkAt(2), containerTok, std::move(value), tokenlist, errorLogger, settings);
                 }
-            } else if (Token::simpleMatch(tok, "+=") && astIsContainer(tok->astOperand1())) {
+
+            } else if (tok->str() == "+=" && astIsContainer(tok->astOperand1())) {
                 const Token* containerTok = tok->astOperand1();
                 const Token* valueTok = tok->astOperand2();
-                MathLib::bigint size = valueFlowGetStrLength(valueTok);
-                if (size == 0)
-                    continue;
-                ValueFlow::Value value(size - 1);
-                value.valueType = ValueFlow::Value::ValueType::CONTAINER_SIZE;
-                value.bound = ValueFlow::Value::Bound::Upper;
-                value.setImpossible();
-                Token* next = nextAfterAstRightmostLeaf(tok);
-                if (!next)
-                    next = tok->next();
-                valueFlowForward(next, containerTok, std::move(value), tokenlist, errorLogger, settings);
+                const MathLib::bigint size = valueFlowGetStrLength(valueTok);
+                forwardMinimumContainerSize(size, tok, containerTok);
+
+            } else if (tok->str() == "=" && Token::simpleMatch(tok->astOperand2(), "+") && astIsContainerString(tok)) {
+                const Token* tok2 = tok->astOperand2();
+                MathLib::bigint size = 0;
+                while (Token::simpleMatch(tok2, "+") && tok2->astOperand2()) {
+                    size += valueFlowGetStrLength(tok2->astOperand2());
+                    tok2 = tok2->astOperand1();
+                }
+                size += valueFlowGetStrLength(tok2);
+                forwardMinimumContainerSize(size, tok, tok->astOperand1());
             }
         }
     }
diff --git a/test/testvalueflow.cpp b/test/testvalueflow.cpp
index 8ede4292d3c..85a38b7bd33 100644
--- a/test/testvalueflow.cpp
+++ b/test/testvalueflow.cpp
@@ -6949,6 +6949,42 @@ class TestValueFlow : public TestFixture {
                "    return a[0];\n"
                "}\n";
         ASSERT(!isKnownContainerSizeValue(tokenValues(code, "a [ 0"), 0).empty());
+
+        code = "void f(const std::string& a) {\n" // #12994
+               "    std::string b = a + \"123\";\n"
+               "    if (b.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "b ."), 2));
+
+        code = "void f(const std::string& a) {\n"
+               "    std::string b = \"123\" + a;\n"
+               "    if (b.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "b ."), 2));
+
+        code = "void f(const std::string& a, const std::string& b) {\n"
+               "    std::string c = a + b + \"123\";\n"
+               "    if (c.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "c ."), 2));
+
+        code = "void f(const std::string& a) {\n"
+               "    std::string b = a + \"123\" + \"456\";\n"
+               "    if (b.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "b ."), 5));
+
+        code = "void f(const std::string& a) {\n"
+               "    std::string b = \"123\" + a + \"456\";\n"
+               "    if (b.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "b ."), 5));
+
+        code = "void f(const std::string& a, const std::string& b) {\n"
+               "    std::string c = \"123\" + a + b;\n"
+               "    if (c.empty()) {}\n"
+               "}";
+        ASSERT_EQUALS("", isImpossibleContainerSizeValue(tokenValues(code, "c ."), 2));
     }
 
     void valueFlowContainerElement()