diff --git a/src/ytdl_sub/script/parser.py b/src/ytdl_sub/script/parser.py
index 3284774e..cb6a15f2 100644
--- a/src/ytdl_sub/script/parser.py
+++ b/src/ytdl_sub/script/parser.py
@@ -18,6 +18,7 @@ from ytdl_sub.script.types.variable import FunctionArgument
 from ytdl_sub.script.types.variable import Variable
 from ytdl_sub.script.utils.exception_formatters import ParserExceptionFormatter
 from ytdl_sub.script.utils.exceptions import UNREACHABLE
+from ytdl_sub.script.utils.exceptions import CycleDetected
 from ytdl_sub.script.utils.exceptions import IncompatibleFunctionArguments
 from ytdl_sub.script.utils.exceptions import InvalidSyntaxException
 from ytdl_sub.script.utils.exceptions import UserException
@@ -334,8 +335,14 @@ class _Parser:
 
         while ch := self._read():
             if ch == ")":
+                # Had '(' to indicate there are args
                 if function_args is not None:
-                    # Had '(' to indicate there are args
+                    if self._custom_function_name == function_name:
+                        self._set_highlight_position(function_start_pos)
+                        raise CycleDetected(
+                            f"The custom function %{function_name} cannot call itself."
+                        )
+
                     try:
                         return Function.from_name_and_args(name=function_name, args=function_args)
                     except IncompatibleFunctionArguments:
diff --git a/src/ytdl_sub/script/utils/exceptions.py b/src/ytdl_sub/script/utils/exceptions.py
index 4c597573..39f9cb79 100644
--- a/src/ytdl_sub/script/utils/exceptions.py
+++ b/src/ytdl_sub/script/utils/exceptions.py
@@ -19,6 +19,10 @@ class FunctionDoesNotExist(UserException):
     """Tried to use a function that does not exist"""
 
 
+class CycleDetected(UserException):
+    """A cycle exists within a user's script"""
+
+
 class FunctionRuntimeException(ValueError):
     """Exception thrown when a ytdl-sub function has an error occur at runtime"""
 
diff --git a/tests/unit/script/types/test_function.py b/tests/unit/script/types/test_function.py
index 4e5bf55e..4ebb206b 100644
--- a/tests/unit/script/types/test_function.py
+++ b/tests/unit/script/types/test_function.py
@@ -8,6 +8,7 @@ from ytdl_sub.script.types.array import ResolvedArray
 from ytdl_sub.script.types.resolvable import Boolean
 from ytdl_sub.script.types.resolvable import Integer
 from ytdl_sub.script.types.resolvable import String
+from ytdl_sub.script.utils.exceptions import CycleDetected
 from ytdl_sub.script.utils.exceptions import FunctionDoesNotExist
 from ytdl_sub.script.utils.exceptions import FunctionRuntimeException
 from ytdl_sub.script.utils.exceptions import IncompatibleFunctionArguments
@@ -117,6 +118,14 @@ class TestFunction:
         ):
             Script({"dne": "{%throw}"}).resolve()
 
+    def test_custom_function_cycle(self):
+        with pytest.raises(
+            CycleDetected, match=re.escape("The custom function %cycle_func cannot call itself.")
+        ):
+            Script(
+                {"%cycle_func": "{%mul(%cycle_func(1), $0)}", "output": "{%cycle_func(1)}"}
+            ).resolve()
+
     def test_lambda_with_custom_function(self):
         assert Script(
             {"%times_two": "{%mul($0, 2)}", "wip": "{%array_apply([1, 2, 3], %times_two)}"}