diff --git a/pykeops/pykeops/sandbox/issue_310.py b/pykeops/pykeops/sandbox/issue_310.py
new file mode 100644
index 000000000..bd45abf36
--- /dev/null
+++ b/pykeops/pykeops/sandbox/issue_310.py
@@ -0,0 +1,23 @@
+import numpy as np
+from pykeops.numpy import Genred
+# Simple formula
+formula = "V0"
+variables = ["V0=Vi(4)"]
+# Sum on Vi
+op = Genred(formula, variables, reduction_op="Sum", axis=0)
+# data
+arg = np.random.randn(10, 4)
+# Perform reduction (add a dimension)
+res = op(arg, backend="CPU")
+print(res.shape)
+#> (1, 4)
+# Sum on Vj
+op = Genred(formula, variables, reduction_op="Sum", axis=1)
+# data
+arg = np.random.randn(10, 4)
+# Perform reduction (should do nothing?)
+res = op(arg, backend="CPU")
+print(res.shape)
+#> (10, 4)
+print(sum(res - arg))
+#> array([0., 0., 0., 0.])
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