diff --git a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
index cf7a8b85c9..1376769772 100644
--- a/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
+++ b/src/Rules/Comparison/ImpossibleCheckTypeHelper.php
@@ -17,7 +17,6 @@
 use PHPStan\Type\MixedType;
 use PHPStan\Type\NeverType;
 use PHPStan\Type\ObjectType;
-use PHPStan\Type\StringType;
 use PHPStan\Type\TypeCombinator;
 use PHPStan\Type\TypeUtils;
 use PHPStan\Type\TypeWithClassName;
@@ -82,10 +81,6 @@ public function findSpecifiedType(
 					if (count(TypeUtils::getConstantScalars($argType)) > 0) {
 						return !$argType->toNumber() instanceof ErrorType;
 					}
-
-					if (!(new StringType())->isSuperTypeOf($argType)->no()) {
-						return null;
-					}
 				} elseif ($functionName === 'defined') {
 					return null;
 				} elseif (
diff --git a/src/Type/Php/IsNumericFunctionTypeSpecifyingExtension.php b/src/Type/Php/IsNumericFunctionTypeSpecifyingExtension.php
index 2810aa711d..4d55395276 100644
--- a/src/Type/Php/IsNumericFunctionTypeSpecifyingExtension.php
+++ b/src/Type/Php/IsNumericFunctionTypeSpecifyingExtension.php
@@ -34,6 +34,11 @@ public function specifyTypes(FunctionReflection $functionReflection, FuncCall $n
 			throw new \PHPStan\ShouldNotHappenException();
 		}
 
+		$argType = $scope->getType($node->args[0]->value);
+		if (!(new StringType())->isSuperTypeOf($argType)->no()) {
+			return new SpecifiedTypes([], []);
+		}
+
 		$numericTypes = [
 			new IntegerType(),
 			new FloatType(),
diff --git a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
index a8d2ad7913..9e9be07432 100644
--- a/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
+++ b/tests/PHPStan/Rules/Comparison/ImpossibleCheckTypeFunctionCallRuleTest.php
@@ -206,6 +206,22 @@ public function testImpossibleCheckTypeFunctionCall(): void
 					677,
 					'Because the type is coming from a PHPDoc, you can turn off this check by setting <fg=cyan>treatPhpDocTypesAsCertain: false</> in your <fg=cyan>%configurationFile%</>.',
 				],
+				[
+					'Call to function is_numeric() with \'123\' will always evaluate to true.',
+					692,
+				],
+				[
+					'Call to function is_numeric() with \'blabla\' will always evaluate to false.',
+					693,
+				],
+				[
+					'Call to function assert() with true will always evaluate to true.',
+					700,
+				],
+				[
+					'Call to function is_numeric() with 123|float will always evaluate to true.',
+					700,
+				],
 			]
 		);
 	}
@@ -298,6 +314,10 @@ public function testImpossibleCheckTypeFunctionCallWithoutAlwaysTrue(): void
 					'Call to function method_exists() with \'CheckTypeFunctionCa…\' and \'unknown\' will always evaluate to false.',
 					648,
 				],
+				[
+					'Call to function is_numeric() with \'blabla\' will always evaluate to false.',
+					693,
+				],
 			]
 		);
 	}
diff --git a/tests/PHPStan/Rules/Comparison/data/check-type-function-call.php b/tests/PHPStan/Rules/Comparison/data/check-type-function-call.php
index 87a3e102cf..836e63e804 100644
--- a/tests/PHPStan/Rules/Comparison/data/check-type-function-call.php
+++ b/tests/PHPStan/Rules/Comparison/data/check-type-function-call.php
@@ -682,3 +682,22 @@ public function doFoo($a): bool
 	}
 
 }
+
+class AssertIsNumeric
+{
+
+	public function doFoo(string $str, float $float)
+	{
+		assert(is_numeric($str));
+		assert(is_numeric('123'));
+		assert(is_numeric('blabla'));
+
+		$isNumeric = $float;
+		if (doFoo()) {
+			$isNumeric = 123;
+		}
+
+		assert(is_numeric($isNumeric));
+	}
+
+}