diff --git a/ChangeLog b/ChangeLog
index fe03477cc7..9373ddfe01 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -7,10 +7,9 @@ What's New in Pylint 2.5.0?
 
 Release date: TBA
 
-* Add a check for cases where the second argument to `isinstance`
-  is not a type.
+* Add a check for cases where the second argument to `isinstance` is not a type.
 
-	Close #3308
+  Close #3308
 
 * Add 'notes-rgx' option, to be used for fixme check.
 
diff --git a/pylint/checkers/typecheck.py b/pylint/checkers/typecheck.py
index cc709e95be..7504121275 100644
--- a/pylint/checkers/typecheck.py
+++ b/pylint/checkers/typecheck.py
@@ -678,6 +678,19 @@ def _is_c_extension(module_node):
     )
 
 
+def _is_invalid_isinstance_type(arg):
+    # Return True if we are sure that arg is not a type
+    inferred = utils.safe_infer(arg)
+    if not inferred:
+        # Cannot infer it so skip it.
+        return False
+    if isinstance(inferred, astroid.Tuple):
+        return any(_is_invalid_isinstance_type(elt) for elt in inferred.elts)
+    if isinstance(inferred, astroid.ClassDef):
+        return False
+    return True
+
+
 class TypeChecker(BaseChecker):
     """try to find bugs in the code using type inference
     """
@@ -1177,17 +1190,8 @@ def _check_isinstance_args(self, node):
             # isinstance called with wrong number of args
             return
 
-        def is_not_type(arg):
-            # Return True if we are sure that arg is not a type
-            if isinstance(utils.safe_infer(arg), astroid.FunctionDef):
-                return True
-            if isinstance(arg, astroid.Tuple):
-                return any([is_not_type(elt) for elt in arg.elts])
-
-            return False
-
         second_arg = node.args[1]
-        if is_not_type(second_arg):
+        if _is_invalid_isinstance_type(second_arg):
             self.add_message("isinstance-second-argument-not-valid-type", node=node)
 
     # pylint: disable=too-many-branches,too-many-locals
diff --git a/tests/functional/i/isinstance_second_argument.py b/tests/functional/i/isinstance_second_argument.py
index 71f8fedec2..62a721b687 100644
--- a/tests/functional/i/isinstance_second_argument.py
+++ b/tests/functional/i/isinstance_second_argument.py
@@ -26,3 +26,4 @@ class B(A):
 isinstance(64, hex) # [isinstance-second-argument-not-valid-type]
 isinstance({b: 100}, (hash, dict)) # [isinstance-second-argument-not-valid-type]
 isinstance("string", ((dict, iter), str, (int, bool))) # [isinstance-second-argument-not-valid-type]
+isinstance(int, 1) # [isinstance-second-argument-not-valid-type]
diff --git a/tests/functional/i/isinstance_second_argument.txt b/tests/functional/i/isinstance_second_argument.txt
index 41ba2fd5bf..7893d2658f 100644
--- a/tests/functional/i/isinstance_second_argument.txt
+++ b/tests/functional/i/isinstance_second_argument.txt
@@ -2,3 +2,4 @@ isinstance-second-argument-not-valid-type:25::Second argument of isinstance is n
 isinstance-second-argument-not-valid-type:26::Second argument of isinstance is not a type
 isinstance-second-argument-not-valid-type:27::Second argument of isinstance is not a type
 isinstance-second-argument-not-valid-type:28::Second argument of isinstance is not a type
+isinstance-second-argument-not-valid-type:29::Second argument of isinstance is not a type