diff --git a/src/build/core/CompileDefinition.go b/src/build/core/CompileDefinition.go
index 852fb4f..a731841 100644
--- a/src/build/core/CompileDefinition.go
+++ b/src/build/core/CompileDefinition.go
@@ -48,7 +48,6 @@ func (f *Function) AddVariable(variable *Variable) {
 	variable.Scope = scope
 
 	scope.variables[variable.Name] = variable
-	scope.Reserve(variable.Register)
 	scope.Use(variable.Register)
 }
 
diff --git a/src/build/cpu/Register_test.go b/src/build/cpu/Register_test.go
new file mode 100644
index 0000000..8c4c4ce
--- /dev/null
+++ b/src/build/cpu/Register_test.go
@@ -0,0 +1,13 @@
+package cpu_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/cli/q/src/build/cpu"
+	"git.akyoto.dev/go/assert"
+)
+
+func TestRegisterString(t *testing.T) {
+	register := cpu.Register(1)
+	assert.Equal(t, "r1", register.String())
+}
diff --git a/src/build/cpu/State.go b/src/build/cpu/State.go
index af1637e..fb8659b 100644
--- a/src/build/cpu/State.go
+++ b/src/build/cpu/State.go
@@ -35,7 +35,7 @@ func (s *State) Use(reg Register) {
 // FindFree tries to find a free register in the given slice of registers.
 func (s *State) FindFree(registers []Register) (Register, bool) {
 	for _, reg := range registers {
-		if !s.IsReserved(reg) {
+		if !s.IsReserved(reg) && !s.IsUsed(reg) {
 			return reg, true
 		}
 	}
diff --git a/src/build/cpu/State_test.go b/src/build/cpu/State_test.go
new file mode 100644
index 0000000..90cdab7
--- /dev/null
+++ b/src/build/cpu/State_test.go
@@ -0,0 +1,45 @@
+package cpu_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/cli/q/src/build/cpu"
+	"git.akyoto.dev/go/assert"
+)
+
+func TestRegisterState(t *testing.T) {
+	s := cpu.State{}
+	assert.False(t, s.IsReserved(0))
+	assert.False(t, s.IsUsed(0))
+	s.Reserve(0)
+	assert.True(t, s.IsReserved(0))
+	assert.False(t, s.IsUsed(0))
+	s.Use(0)
+	assert.True(t, s.IsReserved(0))
+	assert.True(t, s.IsUsed(0))
+	s.Free(0)
+	assert.False(t, s.IsReserved(0))
+	assert.False(t, s.IsUsed(0))
+}
+
+func TestFindFree(t *testing.T) {
+	s := cpu.State{}
+	s.Reserve(0)
+	s.Use(1)
+
+	reg, found := s.FindFree([]cpu.Register{0, 1, 2, 3})
+	assert.True(t, found)
+	assert.Equal(t, reg, 2)
+
+	_, found = s.FindFree([]cpu.Register{0, 1})
+	assert.False(t, found)
+}
+
+func TestMustFindFree(t *testing.T) {
+	s := cpu.State{}
+	s.Reserve(0)
+	s.Use(1)
+
+	reg := s.MustFindFree([]cpu.Register{0, 1, 2, 3})
+	assert.Equal(t, reg, 2)
+}
diff --git a/src/build/fs/Walk_test.go b/src/build/fs/Walk_test.go
index d3f6e17..82651ce 100644
--- a/src/build/fs/Walk_test.go
+++ b/src/build/fs/Walk_test.go
@@ -19,7 +19,12 @@ func TestWalk(t *testing.T) {
 	assert.Contains(t, files, "Walk_test.go")
 }
 
-func TestNonExisting(t *testing.T) {
+func TestWalkFile(t *testing.T) {
+	err := fs.Walk("Walk.go", func(file string) {})
+	assert.NotNil(t, err)
+}
+
+func TestWalkNonExisting(t *testing.T) {
 	err := fs.Walk("does-not-exist", func(file string) {})
 	assert.NotNil(t, err)
 }