diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6d6231f..c2a3daa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
 ### Changed
+- Bug fix: Fixed ACI encoder to handle `-` unary operator
 ### Removed
 
 ## [4.2.0] - 2020-01-15
diff --git a/src/aeso_aci.erl b/src/aeso_aci.erl
index 274be69..26372a8 100644
--- a/src/aeso_aci.erl
+++ b/src/aeso_aci.erl
@@ -194,6 +194,8 @@ encode_expr({bytes, _, B})  ->
 encode_expr({Lit, _, L}) when Lit == oracle_pubkey; Lit == oracle_query_id;
                               Lit == contract_pubkey; Lit == account_pubkey ->
     aeser_api_encoder:encode(Lit, L);
+encode_expr({app, _, {'-', _}, [{int, _, N}]}) ->
+  encode_expr({int, [], -N});
 encode_expr({app, _, F, As}) ->
     Ef = encode_expr(F),
     Eas = encode_exprs(As),