diff --git a/src/core/CompileAssignArray.go b/src/core/CompileAssignArray.go
index fff34ca..217e010 100644
--- a/src/core/CompileAssignArray.go
+++ b/src/core/CompileAssignArray.go
@@ -30,31 +30,31 @@ func (f *Function) CompileAssignArray(node *ast.Assign) error {
 		Length:         byte(1),
 	}
 
-	index := left.Children[1]
+	indexExpr := left.Children[1]
 
-	if index.Token.IsNumeric() {
-		offset, err := f.ToNumber(index.Token)
+	if indexExpr.Token.IsNumeric() {
+		index, err := f.ToNumber(indexExpr.Token)
 
 		if err != nil {
 			return err
 		}
 
-		memory.Offset = int8(offset)
+		memory.Offset = int8(index)
 	} else {
-		typ, indexRegister, isTemporary, err := f.Evaluate(index)
+		index, isTemporary, err := f.Evaluate(indexExpr)
 
 		if err != nil {
 			return err
 		}
 
-		if !types.Is(typ, types.AnyInt) {
-			return errors.New(&errors.TypeMismatch{Encountered: typ.Name(), Expected: types.AnyInt.Name()}, f.File, index.Token.Position)
+		if !types.Is(index.Type, types.AnyInt) {
+			return errors.New(&errors.TypeMismatch{Encountered: index.Type.Name(), Expected: types.AnyInt.Name()}, f.File, indexExpr.Token.Position)
 		}
 
-		memory.OffsetRegister = indexRegister
+		memory.OffsetRegister = index.Register
 
 		if isTemporary {
-			defer f.FreeRegister(indexRegister)
+			defer f.FreeRegister(index.Register)
 		}
 	}
 
diff --git a/src/core/CompileAssignDivision.go b/src/core/CompileAssignDivision.go
index cbca794..bf483eb 100644
--- a/src/core/CompileAssignDivision.go
+++ b/src/core/CompileAssignDivision.go
@@ -58,20 +58,24 @@ func (f *Function) CompileAssignDivision(expr *expression.Expression) error {
 		defer f.UseVariable(remainderVariable)
 	}
 
-	dividend := right.Children[0]
-	_, dividendRegister, isTemporary, err := f.Evaluate(dividend)
+	dividendExpr := right.Children[0]
+	dividend, isTemporary, err := f.Evaluate(dividendExpr)
 
 	if err != nil {
 		return err
 	}
 
+	if !types.Is(dividend.Type, types.AnyInt) {
+		return errors.New(&errors.TypeMismatch{Encountered: dividend.Type.Name(), Expected: types.AnyInt.Name()}, f.File, dividendExpr.Token.Position)
+	}
+
 	divisor := right.Children[1]
-	err = f.Execute(right.Token, dividendRegister, divisor)
+	err = f.Execute(right.Token, dividend.Register, divisor)
 	f.RegisterRegister(asm.MOVE, quotientVariable.Register, x86.RAX)
 	f.RegisterRegister(asm.MOVE, remainderVariable.Register, x86.RDX)
 
 	if isTemporary {
-		f.FreeRegister(dividendRegister)
+		f.FreeRegister(dividend.Register)
 	}
 
 	return err
diff --git a/src/core/CompileFor.go b/src/core/CompileFor.go
index 9f77413..d528dd7 100644
--- a/src/core/CompileFor.go
+++ b/src/core/CompileFor.go
@@ -4,8 +4,10 @@ import (
 	"git.urbach.dev/cli/q/src/asm"
 	"git.urbach.dev/cli/q/src/ast"
 	"git.urbach.dev/cli/q/src/cpu"
+	"git.urbach.dev/cli/q/src/errors"
 	"git.urbach.dev/cli/q/src/expression"
 	"git.urbach.dev/cli/q/src/token"
+	"git.urbach.dev/cli/q/src/types"
 )
 
 // CompileFor compiles a for loop.
@@ -66,18 +68,22 @@ func (f *Function) CompileFor(loop *ast.For) error {
 		f.AddLabel(label)
 		f.RegisterNumber(asm.COMPARE, counter, number)
 	} else {
-		_, register, isTemporary, err := f.Evaluate(to)
+		value, isTemporary, err := f.Evaluate(to)
 
 		if err != nil {
 			return err
 		}
 
+		if !types.Is(value.Type, types.AnyInt) {
+			return errors.New(&errors.TypeMismatch{Encountered: value.Type.Name(), Expected: types.AnyInt.Name()}, f.File, to.Token.Position)
+		}
+
 		if isTemporary {
-			defer f.FreeRegister(register)
+			defer f.FreeRegister(value.Register)
 		}
 
 		f.AddLabel(label)
-		f.RegisterRegister(asm.COMPARE, counter, register)
+		f.RegisterRegister(asm.COMPARE, counter, value.Register)
 	}
 
 	f.Jump(asm.JGE, labelEnd)
diff --git a/src/core/CompileLen.go b/src/core/CompileLen.go
index 4571c38..9eaeb40 100644
--- a/src/core/CompileLen.go
+++ b/src/core/CompileLen.go
@@ -13,21 +13,21 @@ var _len = Function{OutputTypes: []types.Type{types.AnyInt}}
 
 // CompileLen returns the length of a slice.
 func (f *Function) CompileLen(root *expression.Expression) error {
-	typ, register, isTemporary, err := f.Evaluate(root.Children[1])
+	value, isTemporary, err := f.Evaluate(root.Children[1])
 
 	if err != nil {
 		return err
 	}
 
-	if !types.Is(typ, types.AnyArray) {
-		return errors.New(&errors.TypeMismatch{Encountered: typ.Name(), Expected: types.AnyArray.Name(), ParameterName: "array"}, f.File, root.Children[1].Token.Position)
+	if !types.Is(value.Type, types.AnyArray) {
+		return errors.New(&errors.TypeMismatch{Encountered: value.Type.Name(), Expected: types.AnyArray.Name(), ParameterName: "array"}, f.File, root.Children[1].Token.Position)
 	}
 
 	f.SaveRegister(f.CPU.Output[0])
-	f.MemoryRegister(asm.LOAD, asm.Memory{Base: register, Offset: -8, OffsetRegister: math.MaxUint8, Length: 8}, f.CPU.Output[0])
+	f.MemoryRegister(asm.LOAD, asm.Memory{Base: value.Register, Offset: -8, OffsetRegister: math.MaxUint8, Length: 8}, f.CPU.Output[0])
 
 	if isTemporary {
-		f.FreeRegister(register)
+		f.FreeRegister(value.Register)
 	}
 
 	return nil
diff --git a/src/core/Evaluate.go b/src/core/Evaluate.go
index 7189cf6..5f11a75 100644
--- a/src/core/Evaluate.go
+++ b/src/core/Evaluate.go
@@ -2,26 +2,24 @@ package core
 
 import (
 	"git.urbach.dev/cli/q/src/ast"
-	"git.urbach.dev/cli/q/src/cpu"
 	"git.urbach.dev/cli/q/src/errors"
 	"git.urbach.dev/cli/q/src/expression"
 	"git.urbach.dev/cli/q/src/token"
-	"git.urbach.dev/cli/q/src/types"
 )
 
 // Evaluate evaluates an expression and returns a register that contains the value of the expression.
-func (f *Function) Evaluate(expr *expression.Expression) (types.Type, cpu.Register, bool, error) {
+func (f *Function) Evaluate(expr *expression.Expression) (Value, bool, error) {
 	if expr.Token.Kind == token.Identifier {
 		name := expr.Token.Text(f.File.Bytes)
 		variable := f.VariableByName(name)
 
 		if variable == nil {
-			return nil, 0, false, errors.New(&errors.UnknownIdentifier{Name: name}, f.File, expr.Token.Position)
+			return Value{}, false, errors.New(&errors.UnknownIdentifier{Name: name}, f.File, expr.Token.Position)
 		}
 
 		if variable.Alive == 1 {
 			f.UseVariable(variable)
-			return variable.Type, variable.Register, false, nil
+			return Value{variable.Type, variable.Register}, false, nil
 		}
 	}
 
@@ -29,13 +27,13 @@ func (f *Function) Evaluate(expr *expression.Expression) (types.Type, cpu.Regist
 		types, err := f.CompileCall(expr)
 
 		if err != nil {
-			return nil, 0, false, err
+			return Value{}, false, err
 		}
 
-		return types[0], f.CPU.Output[0], false, nil
+		return Value{types[0], f.CPU.Output[0]}, false, nil
 	}
 
 	tmp := f.NewRegister()
 	typ, err := f.ExpressionToRegister(expr, tmp)
-	return typ, tmp, true, err
+	return Value{typ, tmp}, true, err
 }
diff --git a/src/core/ExpressionToMemory.go b/src/core/ExpressionToMemory.go
index 8886f6e..63ba85b 100644
--- a/src/core/ExpressionToMemory.go
+++ b/src/core/ExpressionToMemory.go
@@ -52,17 +52,17 @@ func (f *Function) ExpressionToMemory(node *expression.Expression, memory asm.Me
 		}
 	}
 
-	typ, register, isTemporary, err := f.Evaluate(node)
+	value, isTemporary, err := f.Evaluate(node)
 
 	if err != nil {
 		return nil, err
 	}
 
-	f.MemoryRegister(asm.STORE, memory, register)
+	f.MemoryRegister(asm.STORE, memory, value.Register)
 
 	if isTemporary {
-		f.FreeRegister(register)
+		f.FreeRegister(value.Register)
 	}
 
-	return typ, err
+	return value.Type, err
 }
diff --git a/src/core/Value.go b/src/core/Value.go
new file mode 100644
index 0000000..28b9f81
--- /dev/null
+++ b/src/core/Value.go
@@ -0,0 +1,12 @@
+package core
+
+import (
+	"git.urbach.dev/cli/q/src/cpu"
+	"git.urbach.dev/cli/q/src/types"
+)
+
+// Value combines a register with its data type.
+type Value struct {
+	Type     types.Type
+	Register cpu.Register
+}
diff --git a/src/types/Is.go b/src/types/Is.go
index 875df1a..4728d6a 100644
--- a/src/types/Is.go
+++ b/src/types/Is.go
@@ -27,8 +27,19 @@ func Is(a Type, b Type) bool {
 
 	if a == AnyInt {
 		switch b {
-		case Int64, Int32, Int16, Int8, UInt64, UInt32, UInt16, UInt8:
+		case Int64, Int32, Int16, Int8, UInt64, UInt32, UInt16, UInt8, AnyInt:
 			return true
+		default:
+			return false
+		}
+	}
+
+	if b == AnyInt {
+		switch a {
+		case Int64, Int32, Int16, Int8, UInt64, UInt32, UInt16, UInt8, AnyInt:
+			return true
+		default:
+			return false
 		}
 	}