diff --git a/src/backscattering_experimental_data/metadata/injection.py b/src/backscattering_experimental_data/metadata/injection.py
index acde7e1..aaede05 100644
--- a/src/backscattering_experimental_data/metadata/injection.py
+++ b/src/backscattering_experimental_data/metadata/injection.py
@@ -77,7 +77,16 @@ class SinInjection(Injection):
 
     def to_hdf5(self, store: File | Group) -> Group | File:
         store.attrs["type"] = "sin"
-        store.attrs["amplitude"] = self.amplitude.to_string()
+        if type(self.amplitude) is Quantity:
+            store.attrs["amplitude"] = self.amplitude.to_string()
+        elif type(self.amplitude) is float:  # allowed (dimensionless)
+            store.attrs["amplitude"] = self.amplitude
+        else:
+            raise ValueError(
+                "amplitude must be a dimensionless quantity, not {}".format(
+                    type(self.amplitude)
+                )
+            )
         store.attrs["frequency"] = self.frequency.to_string()
         return store