diff --git a/aviary/subsystems/mass/flops_based/engine.py b/aviary/subsystems/mass/flops_based/engine.py
index 5778937c5f..2bbfc9490a 100644
--- a/aviary/subsystems/mass/flops_based/engine.py
+++ b/aviary/subsystems/mass/flops_based/engine.py
@@ -44,7 +44,7 @@ def compute(self, inputs, outputs):
         scaled_sls_thrust = np.array(inputs[Aircraft.Engine.SCALED_SLS_THRUST])
         scaling_parameter = np.array(inputs[Aircraft.Engine.MASS_SCALER])
 
-        scale_idx = np.where(scale_mass)
+        scale_idx = np.where(np.atleast_1d(scale_mass))
         # indices where scaling is applied and scaling equation is used
         param_idx = np.where(scaling_parameter[scale_idx] >= 0.3)
 
@@ -100,7 +100,7 @@ def compute_partials(self, inputs, J):
 
         # engine mass derivatives
         # indices where scaling is applied
-        scale_idx = np.where(scale_mass)
+        scale_idx = np.where(np.atleast_1d(scale_mass))
         # indices where scaling is applied and scaling equation is used
         param_idx = np.where(scaling_parameter[scale_idx] >= 0.3)