diff --git a/dangerzone/document.py b/dangerzone/document.py
index 415b92d..ca004e8 100644
--- a/dangerzone/document.py
+++ b/dangerzone/document.py
@@ -109,7 +109,10 @@ class Document:
 
     @suffix.setter
     def suffix(self, suf: str) -> None:
-        self._suffix = suf
+        if self._output_filename is None:
+            self._suffix = suf
+        else:
+            raise errors.SuffixNotApplicableException()
 
     @property
     def default_output_filename(self) -> str:
diff --git a/dangerzone/errors.py b/dangerzone/errors.py
index e012235..0c2d3dd 100644
--- a/dangerzone/errors.py
+++ b/dangerzone/errors.py
@@ -71,6 +71,13 @@ class OutputDirIsNotDirException(DocumentFilenameException):
         super().__init__("Specified output directory is actually not a directory")
 
 
+class SuffixNotApplicableException(DocumentFilenameException):
+    """Exception for when the suffix cannot be applied to the output filename."""
+
+    def __init__(self) -> None:
+        super().__init__("Cannot set a suffix after setting an output filename")
+
+
 def handle_document_errors(func: F) -> F:
     """Log document-related errors and exit gracefully."""
 
diff --git a/tests/test_document.py b/tests/test_document.py
index 264bcdd..1cff181 100644
--- a/tests/test_document.py
+++ b/tests/test_document.py
@@ -113,6 +113,10 @@ def test_set_output_filename_suffix(sample_doc: str) -> None:
     d.suffix = safe_extension
     assert d.output_filename.endswith(safe_extension)
 
+    d.output_filename = "something_else.pdf"
+    with pytest.raises(errors.SuffixNotApplicableException) as e:
+        d.suffix = "-new-trusted.pdf"
+
 
 def test_is_unconverted_by_default(sample_doc: str) -> None:
     d = Document(sample_doc)