diff --git a/tools/isledecomp/isledecomp/parser/error.py b/tools/isledecomp/isledecomp/parser/error.py
index b838a394..9ced1498 100644
--- a/tools/isledecomp/isledecomp/parser/error.py
+++ b/tools/isledecomp/isledecomp/parser/error.py
@@ -78,6 +78,11 @@ class ParserError(Enum):
     # they annotate are different.
     WRONG_STRING = 205
 
+    # ERROR: This lineref FUNCTION marker is next to a function declaration or
+    # forward reference. The correct place for the marker is where the function
+    # is implemented so we can match with the PDB.
+    NO_IMPLEMENTATION = 206
+
 
 @dataclass
 class ParserAlert:
diff --git a/tools/isledecomp/isledecomp/parser/parser.py b/tools/isledecomp/isledecomp/parser/parser.py
index c3c9cde3..2e5daf07 100644
--- a/tools/isledecomp/isledecomp/parser/parser.py
+++ b/tools/isledecomp/isledecomp/parser/parser.py
@@ -487,6 +487,9 @@ def read_line(self, line: str):
                 ):
                     self._function_starts_here()
                     self._function_done()
+                elif self.function_sig.endswith(");"):
+                    # Detect forward reference or declaration
+                    self._syntax_error(ParserError.NO_IMPLEMENTATION)
                 else:
                     self.state = ReaderState.WANT_CURLY
 
diff --git a/tools/isledecomp/tests/test_parser.py b/tools/isledecomp/tests/test_parser.py
index 1abae670..772a39c4 100644
--- a/tools/isledecomp/tests/test_parser.py
+++ b/tools/isledecomp/tests/test_parser.py
@@ -695,3 +695,19 @@ def test_static_variable_incomplete_coverage(parser):
     # Failed for TEST module
     assert len(parser.alerts) == 1
     assert parser.alerts[0].code == ParserError.ORPHANED_STATIC_VARIABLE
+
+
+def test_header_function_declaration(parser):
+    """This is either a forward reference or a declaration in a header file.
+    Meaning: The implementation is not here. This is not the correct place
+    for the FUNCTION marker and it will probably not match anything."""
+
+    parser.read_lines(
+        [
+            "// FUNCTION: HELLO 0x1234",
+            "void sample_function(int);",
+        ]
+    )
+
+    assert len(parser.alerts) == 1
+    assert parser.alerts[0].code == ParserError.NO_IMPLEMENTATION