diff --git a/Lib/fontTools/varLib/interpolatableHelpers.py b/Lib/fontTools/varLib/interpolatableHelpers.py
index 2a3540fff..f71b32afd 100644
--- a/Lib/fontTools/varLib/interpolatableHelpers.py
+++ b/Lib/fontTools/varLib/interpolatableHelpers.py
@@ -143,6 +143,9 @@ def min_cost_perfect_bipartite_matching_scipy(G):
     n = len(G)
     rows, cols = linear_sum_assignment(G)
     assert (rows == list(range(n))).all()
+    # Convert numpy array and integer to Python types,
+    # to ensure that this is JSON-serializable.
+    cols = list(int(e) for e in cols)
     return list(cols), matching_cost(G, cols)