diff --git a/tiled/server/pydantic_array.py b/tiled/server/pydantic_array.py
index 952a5c8ba..4e466f7fb 100644
--- a/tiled/server/pydantic_array.py
+++ b/tiled/server/pydantic_array.py
@@ -28,7 +28,7 @@ class BuiltinDtype(BaseModel):
     endianness: Endianness
     kind: Kind
     itemsize: int
-    units: str = ""
+    units: Optional[str] = None
 
     __endianness_map = {
         ">": "big",
@@ -43,7 +43,7 @@ class BuiltinDtype(BaseModel):
     def from_numpy_dtype(cls, dtype) -> "BuiltinDtype":
         # Extract datetime units from the dtype string representation,
         # e.g. `'<M8[ns]'` has `units = 'ns'`.
-        units = ""
+        units = None
         if dtype.kind in ("m", "M"):
             if res := re.search(r"\[(.+)\]$", dtype.str):
                 units = res.group(1)
@@ -79,7 +79,7 @@ def from_json(cls, structure):
             kind=Kind(structure["kind"]),
             itemsize=structure["itemsize"],
             endianness=Endianness(structure["endianness"]),
-            units=structure.get("units", ""),
+            units=structure.get("units"),
         )
 
 
diff --git a/tiled/structures/array.py b/tiled/structures/array.py
index 17fe8db67..e8e06e491 100644
--- a/tiled/structures/array.py
+++ b/tiled/structures/array.py
@@ -77,7 +77,7 @@ class BuiltinDtype:
     endianness: Endianness
     kind: Kind
     itemsize: int
-    units: str = ""
+    units: Optional[str] = None
 
     __endianness_map = {
         ">": "big",
@@ -92,7 +92,7 @@ class BuiltinDtype:
     def from_numpy_dtype(cls, dtype) -> "BuiltinDtype":
         # Extract datetime units from the dtype string representation,
         # e.g. `'<M8[ns]'` has `units = 'ns'`.
-        units = ""
+        units = None
         if dtype.kind in ("m", "M"):
             if res := re.search(r"\[(.+)\]$", dtype.str):
                 units = res.group(1)
@@ -128,7 +128,7 @@ def from_json(cls, structure):
             kind=Kind(structure["kind"]),
             itemsize=structure["itemsize"],
             endianness=Endianness(structure["endianness"]),
-            units=structure.get("units", ""),
+            units=structure.get("units"),
         )