diff --git a/graph/src/data/store/scalar/bigint.rs b/graph/src/data/store/scalar/bigint.rs
index c344ec83a6d..526ca5ba390 100644
--- a/graph/src/data/store/scalar/bigint.rs
+++ b/graph/src/data/store/scalar/bigint.rs
@@ -1,3 +1,4 @@
+use anyhow::bail;
 use num_bigint;
 use serde::{self, Deserialize, Serialize};
 use stable_hash::utils::AsInt;
@@ -207,14 +208,15 @@ impl BigInt {
         }
     }
 
-    pub fn to_unsigned_u256(&self) -> U256 {
+    pub fn to_unsigned_u256(&self) -> Result<U256, anyhow::Error> {
         let (sign, bytes) = self.to_bytes_le();
-        assert!(
-            sign == BigIntSign::NoSign || sign == BigIntSign::Plus,
-            "negative value encountered for U256: {}",
-            self
-        );
-        U256::from_little_endian(&bytes)
+        if sign != BigIntSign::Plus && sign != BigIntSign::NoSign {
+            bail!(
+                "BigInt value is negative, cannot convert to unsigned U256: {}",
+                self
+            );
+        }
+        Ok(U256::from_little_endian(&bytes))
     }
 
     pub fn pow(self, exponent: u8) -> Result<BigInt, anyhow::Error> {
diff --git a/graph/src/data_source/common.rs b/graph/src/data_source/common.rs
index 344253cebdf..f89739c6ab7 100644
--- a/graph/src/data_source/common.rs
+++ b/graph/src/data_source/common.rs
@@ -668,7 +668,7 @@ impl CallDecl {
             (ParamType::Int(_), Value::BigInt(i)) => Ok(Token::Int(i.to_signed_u256())),
             (ParamType::Uint(_), Value::Int(i)) if *i >= 0 => Ok(Token::Uint((*i).into())),
             (ParamType::Uint(_), Value::BigInt(i)) if i.sign() == Sign::Plus => {
-                Ok(Token::Uint(i.to_unsigned_u256()))
+                Ok(Token::Uint(i.to_unsigned_u256()?))
             }
             (ParamType::Array(inner_type), Value::List(values)) => {
                 self.process_entity_array_values(values, inner_type.as_ref(), param_name)
diff --git a/runtime/test/src/test/abi.rs b/runtime/test/src/test/abi.rs
index 422bd25b2d1..336e8e5eacf 100644
--- a/runtime/test/src/test/abi.rs
+++ b/runtime/test/src/test/abi.rs
@@ -515,7 +515,7 @@ async fn test_abi_big_int(api_version: Version) {
         .await;
     let new_uint: BigInt = module.asc_get(new_uint_obj).unwrap();
     assert_eq!(new_uint, BigInt::from(1_i32));
-    let new_uint = new_uint.to_unsigned_u256();
+    let new_uint = new_uint.to_unsigned_u256().unwrap();
     assert_eq!(new_uint, U256([1, 0, 0, 0]));
 
     // Test passing in -50 and increment it by 1
diff --git a/runtime/wasm/src/to_from/external.rs b/runtime/wasm/src/to_from/external.rs
index ca9f994d8a9..022c07b0bc8 100644
--- a/runtime/wasm/src/to_from/external.rs
+++ b/runtime/wasm/src/to_from/external.rs
@@ -243,7 +243,10 @@ impl FromAscObj<AscEnum<EthereumValueKind>> for ethabi::Token {
             EthereumValueKind::Uint => {
                 let ptr: AscPtr<AscBigInt> = AscPtr::from(payload);
                 let n: BigInt = asc_get(heap, ptr, gas, depth)?;
-                Token::Uint(n.to_unsigned_u256())
+                let uint = n
+                    .to_unsigned_u256()
+                    .map_err(DeterministicHostError::Other)?;
+                Token::Uint(uint)
             }
             EthereumValueKind::String => {
                 let ptr: AscPtr<AscString> = AscPtr::from(payload);