diff --git a/CHANGELOG.md b/CHANGELOG.md
index c234d0a4..c596e902 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,9 @@
 
 ### Fixed
 
+- Fixed a crash (`re.error: invalid group reference`) when a source file defines
+  a function-like macro with no arguments, e.g. `#define ok() ...`
+  ([#486](https://github.com/fortran-lang/fortls/issues/486))
 - Fixed missing registered capability for `textDocument/documentHighlight`
   ([#421](https://github.com/fortran-lang/fortls/issues/421s))
 
diff --git a/fortls/parsers/internal/parser.py b/fortls/parsers/internal/parser.py
index 1dc93760..9f95264b 100644
--- a/fortls/parsers/internal/parser.py
+++ b/fortls/parsers/internal/parser.py
@@ -2086,11 +2086,17 @@ def replace_vars(line: str):
 
     def expand_func_macro(def_name: str, def_value: tuple[str, str]):
         def_args, sub = def_value
-        def_args = def_args.split(",")
+        # "".split(",") is [""], not [], so a zero-argument macro such as
+        # `#define ok() ...` used to be treated as having one unnamed argument.
+        # The substitution below then ran `\b()\b`, which matches at every word
+        # boundary, and peppered the replacement text with group references --
+        # `ie/=0` became `...\10`, i.e. a reference to group 10, and re raised
+        # "invalid group reference". Drop empty names so the count is honest.
+        def_args = [arg for arg in (a.strip() for a in def_args.split(",")) if arg]
         regex = re.compile(rf"\b{def_name}\s*\({','.join(['(.*)']*len(def_args))}\)")
 
         for i, arg in enumerate(def_args, start=1):
-            sub = re.sub(rf"\b({arg.strip()})\b", rf"\\{i}", sub)
+            sub = re.sub(rf"\b({re.escape(arg)})\b", rf"\\{i}", sub)
 
         return regex, sub
 
diff --git a/test/test_preproc_parser.py b/test/test_preproc_parser.py
index c897669a..f5adff67 100644
--- a/test/test_preproc_parser.py
+++ b/test/test_preproc_parser.py
@@ -36,3 +36,48 @@ def test_pp_macro_expansion():
     ]
     output, _, _, _ = preprocess_file(lines)
     assert output == ref
+
+
+def test_pp_zero_argument_function_macro():
+    r"""A function-like macro with no arguments expands without crashing.
+
+    Regression test for #486. `"".split(",")` is `[""]`, not `[]`, so a macro
+    such as `ok()` was treated as having one unnamed argument. The argument
+    substitution then ran `\b()\b`, which matches at every word boundary, and
+    injected group references throughout the replacement text -- `ie/=0` became
+    `...\10`, and `re` raised "invalid group reference 10".
+    """
+    lines = [
+        "#define ok() if(ie/=0) then; return; end if;",
+        "subroutine b",
+        "integer :: ie",
+        "ie = 1",
+        "ok()",
+        "end subroutine",
+    ]
+    ref = [
+        "#define ok() if(ie/=0) then; return; end if;",
+        "subroutine b",
+        "integer :: ie",
+        "ie = 1",
+        "if(ie/=0) then; return; end if;",
+        "end subroutine",
+    ]
+    output, _, _, _ = preprocess_file(lines)
+    assert output == ref
+
+
+def test_pp_function_macro_arities():
+    """Zero, one and two argument macros all expand correctly."""
+    lines = [
+        "#define NOARG() 42",
+        "#define SQUARE(x) ((x)*(x))",
+        "#define ADD(a, b) ((a) + (b))",
+        "i = NOARG()",
+        "j = SQUARE(3)",
+        "k = ADD(1, 2)",
+    ]
+    output, _, _, _ = preprocess_file(lines)
+    # The leading space in "( 2)" is pre-existing behaviour: argument capture
+    # does not strip whitespace. Asserted as-is so this test stays about arity.
+    assert output[3:] == ["i = 42", "j = ((3)*(3))", "k = ((1) + ( 2))"]