diff --git a/hal/instructions.go b/hal/instructions.go
index 72eea09..53c6808 100644
--- a/hal/instructions.go
+++ b/hal/instructions.go
@@ -12,7 +12,7 @@ type ProgrammedInstruction struct {
 
 func (pi *ProgrammedInstruction) Execute(module *Module) error {
 	if pi.Instruction.ExecuteWithOperand != nil {
-		module.debug("Operand: %d", pi.Operand)
+		module.debug("Operand: %f", pi.Operand)
 		return pi.Instruction.ExecuteWithOperand(module, pi.Operand)
 	} else if pi.Instruction.Execute != nil {
 		return pi.Instruction.Execute(module)
diff --git a/hal/module.go b/hal/module.go
index fbf4532..342b951 100644
--- a/hal/module.go
+++ b/hal/module.go
@@ -38,21 +38,27 @@ func (h *Module) debug(format string, args ...interface{}) {
 	fmt.Printf("[DEBUG] "+format+"\n", args...)
 }
 
-func (h *Module) Step() {
+func (h *Module) Step() error {
 	instruction := h.programStorage[h.programCounter()]
 	h.increaseProgramCounter()
 	if instruction == nil {
-		return
+		h.debug("Skip undefined instruction %d", h.programCounter())
+		return nil
 	}
 	h.debug("Instruction: %s", instruction.Instruction.Name)
 	h.debug("Accumulator before: %f", h.accumulator)
-	instruction.Execute(h)
+	if err := instruction.Execute(h); err != nil {
+		return err
+	}
 	h.debug("Accumulator after: %f", h.accumulator)
+	return nil
 }
 
 func (h *Module) Run() error {
 	for !h.isStopped && h.programCounter() <= maxInstructions {
-		h.Step()
+		if err := h.Step(); err != nil {
+			return err
+		}
 	}
 	if h.programCounter() > maxInstructions {
 		return fmt.Errorf("module exceeded max instructions without being stopped")
diff --git a/parser/program_parser.go b/parser/program_parser.go
index 563cb86..24a456c 100644
--- a/parser/program_parser.go
+++ b/parser/program_parser.go
@@ -29,7 +29,7 @@ func ParseProgram(input []string) (hal.Program, error) {
 }
 
 func parseInstruction(args []string) (*hal.ProgrammedInstruction, error) {
-	if len(args) != 0 && len(args) != 1 {
+	if len(args) != 1 && len(args) != 2 {
 		return nil, fmt.Errorf("invalid instruction args count (count = %d)", len(args))
 	}
 	instruction := hal.FindInstructionByName(args[0])