diff --git a/changelog.d/18112.bugfix b/changelog.d/18112.bugfix
new file mode 100644
index 00000000000..61c94280d85
--- /dev/null
+++ b/changelog.d/18112.bugfix
@@ -0,0 +1 @@
+Raise an error if someone is using an incorrect suffix in a config duration string.
diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index adce34c03a8..912346a423d 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -221,9 +221,13 @@ def parse_duration(value: Union[str, int]) -> int:
             The number of milliseconds in the duration.
 
         Raises:
-            TypeError, if given something other than an integer or a string
+            TypeError: if given something other than an integer or a string, or the
+                duration is using an incorrect suffix.
             ValueError: if given a string not of the form described above.
         """
+        # For integers, we prefer to use `type(value) is int` instead of
+        # `isinstance(value, int)` because we want to exclude subclasses of int, such as
+        # bool.
         if type(value) is int:  # noqa: E721
             return value
         elif isinstance(value, str):
@@ -246,9 +250,20 @@ def parse_duration(value: Union[str, int]) -> int:
             if suffix in sizes:
                 value = value[:-1]
                 size = sizes[suffix]
+            elif suffix.isdigit():
+                #  No suffix is treated as milliseconds.
+                value = value
+                size = 1
+            else:
+                raise TypeError(
+                    f"Bad duration suffix {value} (expected no suffix or one of these suffixes: {sizes.keys()})"
+                )
+
             return int(value) * size
         else:
-            raise TypeError(f"Bad duration {value!r}")
+            raise TypeError(
+                f"Bad duration type {value!r} (expected int or string duration)"
+            )
 
     @staticmethod
     def abspath(file_path: str) -> str: