diff --git a/lib/galaxy/tool_util/verify/asserts/image.py b/lib/galaxy/tool_util/verify/asserts/image.py
index d9972994d3f..daf4ab8e70c 100644
--- a/lib/galaxy/tool_util/verify/asserts/image.py
+++ b/lib/galaxy/tool_util/verify/asserts/image.py
@@ -320,19 +320,19 @@ CenterOfMassEps = Annotated[
     ),
 ]
 Labels = Annotated[
-    Optional[Union[str, List[int]]],
+    Optional[Union[str, List[Union[float, int]]]],
     AssertionParameter(
         "List of labels, separated by a comma. Labels *not* on this list will be excluded from consideration. Cannot be used in combination with ``exclude_labels``.",
         xml_type="xs:string",
-        json_type="typing.Optional[typing.List[int]]",
+        json_type=f"typing.Optional[typing.List[{JSON_STRICT_NUMBER}]]",
     ),
 ]
 ExcludeLabels = Annotated[
-    Optional[Union[str, List[int]]],
+    Optional[Union[str, List[Union[float, int]]]],
     AssertionParameter(
         "List of labels to be excluded from consideration, separated by a comma. The primary usage of this attribute is to exclude the background of a label image. Cannot be used in combination with ``labels``.",
         xml_type="xs:string",
-        json_type="typing.Optional[typing.List[int]]",
+        json_type=f"typing.Optional[typing.List[{JSON_STRICT_NUMBER}]]",
     ),
 ]
 MeanObjectSize = Annotated[
@@ -734,8 +734,8 @@ def _get_image_labels(
     channel: Optional[Union[int, str]] = None,
     slice: Optional[Union[int, str]] = None,
     frame: Optional[Union[int, str]] = None,
-    labels: Optional[Union[str, List[int]]] = None,
-    exclude_labels: Optional[Union[str, List[int]]] = None,
+    labels: Labels = None,
+    exclude_labels: ExcludeLabels = None,
 ) -> Tuple["numpy.typing.NDArray", List[Any]]:
     """
     Determines the unique labels in the output image or a specific channel.
@@ -743,7 +743,7 @@ def _get_image_labels(
     assert labels is None or exclude_labels is None
     im_arr = _get_image(output_bytes, channel, slice, frame)
 
-    def cast_label(label):
+    def cast_label(label: str) -> Union[float, int]:
         label = label.strip()
         if numpy.issubdtype(im_arr.dtype, numpy.integer):
             return int(label)
@@ -752,12 +752,12 @@ def _get_image_labels(
         raise AssertionError(f'Unsupported image label type: "{im_arr.dtype}"')
 
     # Determine labels present in the image.
-    present_labels = numpy.unique(im_arr)
+    present_labels: List[Any] = numpy.unique(im_arr).tolist()
 
     # Apply filtering due to `labels` (keep only those).
     if labels is None:
         labels = []
-    if isinstance(labels, str):
+    elif isinstance(labels, str):
         labels = [cast_label(label) for label in labels.split(",") if len(label) > 0]
     if len(labels) > 0:
         present_labels = [label for label in present_labels if label in labels]
@@ -765,7 +765,7 @@ def _get_image_labels(
     # Apply filtering due to `exclude_labels`.
     if exclude_labels is None:
         exclude_labels = []
-    if isinstance(exclude_labels, str):
+    elif isinstance(exclude_labels, str):
         exclude_labels = [cast_label(label) for label in exclude_labels.split(",") if len(label) > 0]
     present_labels = [label for label in present_labels if label not in exclude_labels]