diff --git a/ports/geckolib/properties.mako.rs b/ports/geckolib/properties.mako.rs
index d9fec02b3b1..904181b264d 100644
--- a/ports/geckolib/properties.mako.rs
+++ b/ports/geckolib/properties.mako.rs
@@ -30,7 +30,8 @@ use style::properties::longhands;
 use style::properties::make_cascade_vec;
 use style::properties::style_struct_traits::*;
 use values::{StyleCoordHelpers, ToGeckoStyleCoord, convert_nscolor_to_rgba};
-use values::{convert_rgba_to_nscolor, round_border_to_device_pixels};
+use values::{convert_rgba_to_nscolor, debug_assert_unit_is_safe_to_copy};
+use values::round_border_to_device_pixels;
 
 #[derive(Clone)]
 pub struct GeckoComputedValues {
@@ -285,9 +286,7 @@ def set_gecko_property(ffi_name, expr):
                                &mut self.gecko.${union_ffi_name});
     }
     fn copy_${ident}_from(&mut self, other: &Self) {
-        use gecko_bindings::structs::nsStyleUnit::eStyleUnit_Calc;
-        debug_assert!(self.gecko.${unit_ffi_name} != eStyleUnit_Calc,
-                      "stylo: Can't yet handle refcounted Calc");
+        debug_assert_unit_is_safe_to_copy(self.gecko.${unit_ffi_name});
         self.gecko.${unit_ffi_name} =  other.gecko.${unit_ffi_name};
         self.gecko.${union_ffi_name} = other.gecko.${union_ffi_name};
     }
@@ -305,10 +304,8 @@ def set_gecko_property(ffi_name, expr):
                                         &mut self.gecko.${y_union_ffi_name});
     }
     fn copy_${ident}_from(&mut self, other: &Self) {
-        use gecko_bindings::structs::nsStyleUnit::eStyleUnit_Calc;
-        debug_assert!(self.gecko.${x_unit_ffi_name} != eStyleUnit_Calc &&
-                      self.gecko.${y_unit_ffi_name} != eStyleUnit_Calc,
-                      "stylo: Can't yet handle refcounted Calc");
+        debug_assert_unit_is_safe_to_copy(self.gecko.${x_unit_ffi_name});
+        debug_assert_unit_is_safe_to_copy(self.gecko.${y_unit_ffi_name});
         self.gecko.${x_unit_ffi_name} = other.gecko.${x_unit_ffi_name};
         self.gecko.${x_union_ffi_name} = other.gecko.${x_union_ffi_name};
         self.gecko.${y_unit_ffi_name} = other.gecko.${y_unit_ffi_name};
@@ -559,6 +556,7 @@ fn static_assert() {
         }
     }
     fn copy_z_index_from(&mut self, other: &Self) {
+        debug_assert_unit_is_safe_to_copy(self.gecko.mZIndex.mUnit);
         self.gecko.mZIndex.mUnit = other.gecko.mZIndex.mUnit;
         self.gecko.mZIndex.mValue = other.gecko.mZIndex.mValue;
     }
@@ -672,6 +670,7 @@ fn static_assert() {
         }
     }
     fn copy_vertical_align_from(&mut self, other: &Self) {
+        debug_assert_unit_is_safe_to_copy(self.gecko.mVerticalAlign.mUnit);
         self.gecko.mVerticalAlign.mUnit = other.gecko.mVerticalAlign.mUnit;
         self.gecko.mVerticalAlign.mValue = other.gecko.mVerticalAlign.mValue;
     }
diff --git a/ports/geckolib/values.rs b/ports/geckolib/values.rs
index d9e6da0fd83..7a313315f1f 100644
--- a/ports/geckolib/values.rs
+++ b/ports/geckolib/values.rs
@@ -129,3 +129,7 @@ pub fn round_border_to_device_pixels(width: Au, au_per_device_px: Au) -> Au {
         max(au_per_device_px, Au(width.0 / au_per_device_px.0 * au_per_device_px.0))
     }
 }
+
+pub fn debug_assert_unit_is_safe_to_copy(unit: nsStyleUnit) {
+    debug_assert!(unit != nsStyleUnit::eStyleUnit_Calc, "stylo: Can't yet handle refcounted Calc");
+}