diff --git a/lib/checksizeof.cpp b/lib/checksizeof.cpp
index c12f507b7..4325d35b5 100644
--- a/lib/checksizeof.cpp
+++ b/lib/checksizeof.cpp
@@ -139,6 +139,8 @@ void CheckSizeof::checkSizeofForPointerSize()
             } else if (Token::Match(tok, "memcpy|memcmp|memmove|strncpy|strncmp|strncat (")) {
                 variable = tok->tokAt(2);
                 variable2 = variable->nextArgument();
+                if (!variable2)
+                    continue;
                 tokVar = variable2->nextArgument();
 
             } else {
diff --git a/test/testsizeof.cpp b/test/testsizeof.cpp
index 7fddeee64..bca511067 100644
--- a/test/testsizeof.cpp
+++ b/test/testsizeof.cpp
@@ -40,6 +40,7 @@ private:
         TEST_CASE(sizeofForNumericParameter);
         TEST_CASE(suspiciousSizeofCalculation);
         TEST_CASE(sizeofVoid);
+        TEST_CASE(customStrncat);
     }
 
     void check(const char code[]) {
@@ -635,6 +636,14 @@ private:
         ASSERT_EQUALS("[test.cpp:5]: (portability) '(foo[0]).data' is of type 'void *'. When using void pointers in calculations, the behaviour is undefined.\n", errout.str());
     }
 
+    void customStrncat() {
+        // Ensure we don't crash on custom-defined strncat, ticket #5875
+        check("char strncat ();\n"
+              "int main () {\n"
+              "  return strncat ();\n"
+              "}\n");
+    }
+
 };
 
 REGISTER_TEST(TestSizeof)