diff --git a/rebound/horizons.py b/rebound/horizons.py
index 5de6013ee..0f1aea5f9 100644
--- a/rebound/horizons.py
+++ b/rebound/horizons.py
@@ -7,6 +7,7 @@
 import re
 import warnings
 import sys
+from .units import convert_mass
 
 HORIZONSBASEURL = "https://ssd.jpl.nasa.gov/api/horizons.api?"
 
@@ -65,7 +66,7 @@ def api_request(particle, datestart, dateend, plane):
     return body
 
 
-def query_horizons_for_particle(particle=None, m=None, x=None, y=None, z=None, vx=None, vy=None, vz=None, primary=None, a=None,
+def query_horizons_for_particle(mass_unit=None, particle=None, m=None, x=None, y=None, z=None, vx=None, vy=None, vz=None, primary=None, a=None,
                 anom=None, e=None, omega=None, inc=None, Omega=None, MEAN=None, date=None, plane="ecliptic", hash=0):
     if plane not in ["ecliptic", "frame"]:
         raise AttributeError(
@@ -152,7 +153,11 @@ def query_horizons_for_particle(particle=None, m=None, x=None, y=None, z=None, v
         else:
             print("Found body (Name could not be detected)")
     if m is not None:
-        p.m = m
+        if mass_unit is not None:
+            p.m = convert_mass(m, mass_unit, "kg")
+        else:
+            ## Assume kg
+            p.m = m
     elif idn is not None:
         try:
             p.m = float(
diff --git a/rebound/simulation.py b/rebound/simulation.py
index d194829da..7f03aab96 100644
--- a/rebound/simulation.py
+++ b/rebound/simulation.py
@@ -920,7 +920,8 @@ def add(self, particle=None, **kwargs):
                     if "frame" not in kwargs:
                         if hasattr(self, 'default_plane'):
                             kwargs["plane"] = self.default_plane # allow ASSIST to set default plane
-                    self.add(horizons.query_horizons_for_particle(particle, **kwargs), hash=particle)
+                    mass_unit = hash_to_unit(self.python_unit_m) # For manually provided masses
+                    self.add(horizons.query_horizons_for_particle(mass_unit, particle, **kwargs), hash=particle)
                     units_convert_particle(self.particles[-1], 'km', 's', 'kg', hash_to_unit(self.python_unit_l), hash_to_unit(self.python_unit_t), hash_to_unit(self.python_unit_m))
             else: 
                 raise ValueError("Argument passed to add() not supported.")