diff --git a/PySDM/builder.py b/PySDM/builder.py
index b5f0e0e3fa..eb3c176dec 100644
--- a/PySDM/builder.py
+++ b/PySDM/builder.py
@@ -107,7 +107,7 @@ def build(
                 LiquidSpheres.__name__,
                 MixedPhaseSpheres.__name__,
             ), "implied volume-to-mass conversion is only supported for spherical particles"
-            attributes["water mass"] = (
+            attributes["signed water mass"] = (
                 self.particulator.formulae.particle_shape_and_density.volume_to_mass(
                     attributes.pop("volume")
                 )