diff --git a/setup.py b/setup.py
index 427f08bc6..c7f9470f7 100644
--- a/setup.py
+++ b/setup.py
@@ -152,8 +152,10 @@ GeneralizedIndex = NewType('GeneralizedIndex', int)
 SSZObject = TypeVar('SSZObject', bound=View)
 '''
 SUNDRY_CONSTANTS_FUNCTIONS = '''
-def ceillog2(x: uint64) -> int:
-    return (x - 1).bit_length()
+def ceillog2(x: int) -> uint64:
+    if x < 1:
+        raise ValueError(f"ceillog2 accepts only positive values, x={x}")
+    return uint64((x - 1).bit_length())
 '''
 PHASE0_SUNDRY_FUNCTIONS = '''
 def get_eth1_data(block: Eth1Block) -> Eth1Data: