diff --git a/src/isa/exec.rs b/src/isa/exec.rs
index ee8b9e6..294a21b 100644
--- a/src/isa/exec.rs
+++ b/src/isa/exec.rs
@@ -504,12 +504,21 @@ impl InstructionSet for ArithmeticOp {
                 reg,
                 idx,
                 regs.get(reg, idx).and_then(|val| {
-                    let mut n = Number::from(*step);
-                    debug_assert!(
-                        n.reshape(val.layout()),
-                        "reshape target byte length is always greater"
-                    );
-                    val.int_add(n, IntFlags { signed: false, wrap: false })
+                    if step.as_i8() < 0 {
+                        let mut n = Number::from(-step.as_i8());
+                        debug_assert!(
+                            n.reshape(val.layout()),
+                            "reshape target byte length is always greater"
+                        );
+                        val.int_sub(n, IntFlags { signed: false, wrap: false })
+                    } else {
+                        let mut n = Number::from(*step);
+                        debug_assert!(
+                            n.reshape(val.layout()),
+                            "reshape target byte length is always greater"
+                        );
+                        val.int_add(n, IntFlags { signed: false, wrap: false })
+                    }
                 }),
             ),
             ArithmeticOp::Neg(reg, idx) => {