diff --git a/cashu/core/helpers.py b/cashu/core/helpers.py
index 7c20faa..15b77cd 100644
--- a/cashu/core/helpers.py
+++ b/cashu/core/helpers.py
@@ -53,5 +53,5 @@ def calculate_number_of_blank_outputs(fee_reserve_sat: int):
     """
     assert fee_reserve_sat >= 0, "Fee reserve can't be negative."
     if fee_reserve_sat == 0:
-        return 1
-    return math.ceil(math.log2(fee_reserve_sat))
+        return 0
+    return max(math.ceil(math.log2(fee_reserve_sat)), 1)
diff --git a/tests/test_core.py b/tests/test_core.py
index 3782aa9..470b140 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -43,8 +43,15 @@ def test_calculate_number_of_blank_outputs_for_small_fee_reserve():
     assert n_blank_outputs == expected_n_blank_outputs
 
 
-def test_calculate_number_of_blank_outputs_fails_for_negative_fee_reserve():
+def test_calculate_number_of_blank_outputs_for_zero_fee_reserve():
     # Negative fee reserve is not supported.
     fee_reserve_sat = 0
+    n_blank_outputs = calculate_number_of_blank_outputs(fee_reserve_sat)
+    assert n_blank_outputs == 0
+
+
+def test_calculate_number_of_blank_outputs_fails_for_negative_fee_reserve():
+    # Negative fee reserve is not supported.
+    fee_reserve_sat = -1
     with pytest.raises(AssertionError):
         _ = calculate_number_of_blank_outputs(fee_reserve_sat)