diff --git a/src/ytdl_sub/script/script.py b/src/ytdl_sub/script/script.py
index 771f51ee..7873dd13 100644
--- a/src/ytdl_sub/script/script.py
+++ b/src/ytdl_sub/script/script.py
@@ -9,6 +9,7 @@ from ytdl_sub.script.types.syntax_tree import SyntaxTree
 from ytdl_sub.script.types.variable import Variable
 from ytdl_sub.script.utils.exceptions import UNREACHABLE
 from ytdl_sub.script.utils.exceptions import CycleDetected
+from ytdl_sub.script.utils.exceptions import InvalidCustomFunctionArguments
 from ytdl_sub.script.utils.name_validation import validate_variable_name
 
 # pylint: disable=missing-raises-doc
@@ -93,6 +94,21 @@ class Script:
                 deps=[],
             )
 
+    def _ensure_custom_function_arguments_valid(self):
+        for custom_function_name, custom_function in self._functions.items():
+            indices = sorted([arg.index for arg in custom_function.function_arguments])
+            if indices != list(range(len(indices))):
+                if len(indices) == 1:
+                    raise InvalidCustomFunctionArguments(
+                        f"Custom function %{custom_function_name} has invalid function arguments: "
+                        f"The argument must start with $0, not ${indices[0]}."
+                    )
+                raise InvalidCustomFunctionArguments(
+                    f"Custom function %{custom_function_name} has invalid function arguments: "
+                    f"{', '.join(sorted(f'${idx}' for idx in indices))} "
+                    f"do not increment from $0 to ${len(indices) - 1}."
+                )
+
     def __init__(self, overrides: Dict[str, str]):
         function_names: Set[str] = {
             self._function_name(name) for name in overrides.keys() if self._is_function(name)
@@ -126,6 +142,7 @@ class Script:
         }
 
         self._ensure_no_custom_function_cycles()
+        self._ensure_custom_function_arguments_valid()
         self._ensure_no_variable_cycles()
 
     def resolve(
diff --git a/src/ytdl_sub/script/types/variable.py b/src/ytdl_sub/script/types/variable.py
index 62c68798..1698803c 100644
--- a/src/ytdl_sub/script/types/variable.py
+++ b/src/ytdl_sub/script/types/variable.py
@@ -13,8 +13,10 @@ class Variable(NamedArgument):
 class FunctionArgument(Variable):
     """Arguments for custom functions, i.e. $0, $1, etc"""
 
+    index: int
+
     @classmethod
     def from_idx(cls, idx: int, custom_function_name: Optional[str]) -> "FunctionArgument":
         if custom_function_name:
-            return FunctionArgument(name=f"${custom_function_name}___{idx}")
-        return FunctionArgument(name=f"${idx}")
+            return FunctionArgument(name=f"${custom_function_name}___{idx}", index=idx)
+        return FunctionArgument(name=f"${idx}", index=idx)
diff --git a/src/ytdl_sub/script/utils/exceptions.py b/src/ytdl_sub/script/utils/exceptions.py
index fa67e1f0..dc9e63b6 100644
--- a/src/ytdl_sub/script/utils/exceptions.py
+++ b/src/ytdl_sub/script/utils/exceptions.py
@@ -19,6 +19,10 @@ class InvalidFunctionName(UserException):
     """Custom function name is invalid"""
 
 
+class InvalidCustomFunctionArguments(UserException):
+    """Custom function arguments are invalid (i.e. they do not increment)"""
+
+
 class InvalidCustomFunctionArgumentName(UserException):
     """Custom function argument name (i.e. $0) is invalid"""
 
diff --git a/tests/unit/script/types/test_custom_function.py b/tests/unit/script/types/test_custom_function.py
index 01708f98..649df654 100644
--- a/tests/unit/script/types/test_custom_function.py
+++ b/tests/unit/script/types/test_custom_function.py
@@ -3,11 +3,11 @@ import re
 import pytest
 
 from ytdl_sub.script.script import Script
-from ytdl_sub.script.types.array import ResolvedArray
 from ytdl_sub.script.types.resolvable import Integer
 from ytdl_sub.script.utils.exceptions import CycleDetected
 from ytdl_sub.script.utils.exceptions import FunctionDoesNotExist
 from ytdl_sub.script.utils.exceptions import InvalidCustomFunctionArgumentName
+from ytdl_sub.script.utils.exceptions import InvalidCustomFunctionArguments
 
 
 class TestCustomFunction:
@@ -90,7 +90,7 @@ class TestCustomFunction:
             "$3.14",
         ],
     )
-    def test_custom_function_invalid_function_arguments(self, name: str):
+    def test_custom_function_invalid_function_argument_names(self, name: str):
         with pytest.raises(
             InvalidCustomFunctionArgumentName,
             match=re.escape(
@@ -104,3 +104,46 @@ class TestCustomFunction:
                     "output": "{%func0(1)}",
                 }
             ).resolve()
+
+    @pytest.mark.parametrize(
+        "argument",
+        [
+            "$1",
+            "$2",
+            "$3",
+        ],
+    )
+    def test_custom_function_invalid_function_argument_single(self, argument: str):
+        with pytest.raises(
+            InvalidCustomFunctionArguments,
+            match=re.escape(
+                f"Custom function %func1 has invalid function arguments: "
+                f"The argument must start with $0, not {argument}."
+            ),
+        ):
+            Script(
+                {
+                    "%func1": f"{{[{argument}]}}",
+                }
+            ).resolve()
+
+    @pytest.mark.parametrize(
+        "arguments",
+        [
+            "$0, $2",
+            "$1, $2, $3",
+        ],
+    )
+    def test_custom_function_invalid_function_argument_out_of_order(self, arguments: str):
+        with pytest.raises(
+            InvalidCustomFunctionArguments,
+            match=re.escape(
+                f"Custom function %func1 has invalid function arguments: "
+                f"{arguments} do not increment from $0 to ${len(arguments.split(',')) - 1}."
+            ),
+        ):
+            Script(
+                {
+                    "%func1": f"{{[{arguments}]}}",
+                }
+            ).resolve()