diff --git a/lib/checkbufferoverrun.cpp b/lib/checkbufferoverrun.cpp
index 46d8e1c7c6c..87729ebbb79 100644
--- a/lib/checkbufferoverrun.cpp
+++ b/lib/checkbufferoverrun.cpp
@@ -68,7 +68,9 @@ static const ValueFlow::Value *getBufferSizeValue(const Token *tok)
     auto it = std::find_if(tokenValues.cbegin(), tokenValues.cend(), std::mem_fn(&ValueFlow::Value::isBufferSizeValue));
     if (it != tokenValues.cend())
         return &*it;
-    it = std::find_if(tokenValues.cbegin(), tokenValues.cend(), std::mem_fn(&ValueFlow::Value::isContainerSizeValue));
+    it = std::find_if(tokenValues.cbegin(), tokenValues.cend(), [](const ValueFlow::Value& v) {
+        return v.isContainerSizeValue() && v.isKnown();
+    });
     return it == tokenValues.cend() ? nullptr : &*it;
 }
 
diff --git a/test/testbufferoverrun.cpp b/test/testbufferoverrun.cpp
index 37ab7083902..1f69adb50c4 100644
--- a/test/testbufferoverrun.cpp
+++ b/test/testbufferoverrun.cpp
@@ -3564,6 +3564,14 @@ class TestBufferOverrun : public TestFixture {
               "        memset(&a[i], 0, sizeof(a));\n"
               "}\n");
         ASSERT_EQUALS("[test.cpp:4:16]: (error) Buffer is accessed out of bounds: &a[i] [bufferAccessOutOfBounds]\n", errout_str());
+
+        check("void f(const std::vector<uint8_t>& s) {\n" // #14948
+              "    if (s.size() < 4)\n"
+              "        return;\n"
+              "    uint32_t u = 0;\n"
+              "    std::memcpy(&u, &s[0], sizeof(u));\n"
+              "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void buffer_overrun_errorpath() {