diff --git a/netbox/custom_fields.go b/netbox/custom_fields.go
index 90e11522..7767ee1c 100644
--- a/netbox/custom_fields.go
+++ b/netbox/custom_fields.go
@@ -27,30 +27,28 @@ var customFieldsSchemaRead = &schema.Schema{
 	},
 }
 
-func customFieldsDiff(ctx context.Context, d *schema.ResourceDiff, meta interface{}) error {
-	cfg := d.GetRawConfig().GetAttr(customFieldsKey)
-	cfm, ok := d.Get(customFieldsKey).(map[string]interface{})
-
-	if cfg.IsNull() || !ok {
-		d.SetNew(customFieldsKey, nil)
-		return nil
-	}
+func normalizeCustomFields(cfm map[string]interface{}) map[string]interface{} {
+	newcfm := make(map[string]interface{})
 
 	for k, v := range cfm {
-		if v == "" {
-			delete(cfm, k)
+		if v != nil && v != "" {
+			newcfm[k] = v
 		}
 	}
 
-	d.SetNew(customFieldsKey, cfm)
-	return nil
+	return newcfm
 }
 
-func computeCustomFieldsModel(d *schema.ResourceData) interface{} {
-	oldcf, newcf := d.GetChange(customFieldsKey)
+func mergeCustomFields(oldcfm, newcfm map[string]interface{}) map[string]interface{} {
+	if newcfm == nil {
+		newcfm = make(map[string]interface{})
+	}
 
-	oldcfm, _ := oldcf.(map[string]interface{})
-	newcfm, _ := newcf.(map[string]interface{})
+	for k, v := range newcfm {
+		if v == nil {
+			newcfm[k] = ""
+		}
+	}
 
 	for k := range oldcfm {
 		if _, ok := newcfm[k]; !ok {
@@ -61,15 +59,29 @@ func computeCustomFieldsModel(d *schema.ResourceData) interface{} {
 	return newcfm
 }
 
+func customFieldsDiff(ctx context.Context, d *schema.ResourceDiff, meta interface{}) error {
+	cfg := d.GetRawConfig().GetAttr(customFieldsKey)
+	cfm, ok := d.Get(customFieldsKey).(map[string]interface{})
+
+	if cfg.IsNull() || !ok {
+		d.SetNew(customFieldsKey, nil)
+	} else {
+		newcfm := normalizeCustomFields(cfm)
+		d.SetNew(customFieldsKey, newcfm)
+	}
+
+	return nil
+}
+
 func computeCustomFieldsAttr(cf interface{}) map[string]interface{} {
 	cfm, _ := cf.(map[string]interface{})
-	newcfm := make(map[string]interface{})
+	return normalizeCustomFields(cfm)
+}
 
-	for k, v := range cfm {
-		if vs, _ := v.(string); vs != "" {
-			newcfm[k] = v
-		}
-	}
+func computeCustomFieldsModel(d *schema.ResourceData) interface{} {
+	oldcf, newcf := d.GetChange(customFieldsKey)
 
-	return newcfm
+	oldcfm, _ := oldcf.(map[string]interface{})
+	newcfm, _ := newcf.(map[string]interface{})
+	return mergeCustomFields(oldcfm, newcfm)
 }
diff --git a/netbox/custom_fields_test.go b/netbox/custom_fields_test.go
new file mode 100644
index 00000000..23d9b379
--- /dev/null
+++ b/netbox/custom_fields_test.go
@@ -0,0 +1,185 @@
+package netbox
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCustomFields_normalize(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    map[string]interface{}
+		expected map[string]interface{}
+	}{
+		{
+			name:     "MapNil",
+			input:    nil,
+			expected: map[string]interface{}{},
+		},
+		{
+			name:     "MapEmpty",
+			input:    map[string]interface{}{},
+			expected: map[string]interface{}{},
+		},
+		{
+			name: "MapFull",
+			input: map[string]interface{}{
+				"key1": "value1",
+				"key2": "value2",
+			},
+			expected: map[string]interface{}{
+				"key1": "value1",
+				"key2": "value2",
+			},
+		},
+		{
+			name: "MapMixed",
+			input: map[string]interface{}{
+				"key1": "value1",
+				"key2": "",
+				"key3": nil,
+			},
+			expected: map[string]interface{}{
+				"key1": "value1",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := normalizeCustomFields(tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func TestCustomFields_merge(t *testing.T) {
+	tests := []struct {
+		name     string
+		state    map[string]interface{}
+		input    map[string]interface{}
+		expected map[string]interface{}
+	}{
+		{
+			name:     "MapNilBoth",
+			state:    nil,
+			input:    nil,
+			expected: map[string]interface{}{},
+		},
+		{
+			name:     "MapNilNew",
+			state:    map[string]interface{}{},
+			input:    nil,
+			expected: map[string]interface{}{},
+		},
+		{
+			name:     "MapNilOld",
+			state:    nil,
+			input:    map[string]interface{}{},
+			expected: map[string]interface{}{},
+		},
+		{
+			name: "MapUnsetWithMissing",
+			state: map[string]interface{}{
+				"key1": "value1",
+			},
+			input: map[string]interface{}{},
+			expected: map[string]interface{}{
+				"key1": "",
+			},
+		},
+		{
+			name: "MapUnsetWithNil",
+			state: map[string]interface{}{
+				"key1": "value1",
+			},
+			input: map[string]interface{}{
+				"key1": nil,
+			},
+			expected: map[string]interface{}{
+				"key1": "",
+			},
+		},
+		{
+			name: "MapUnsetWithEmpty",
+			state: map[string]interface{}{
+				"key1": "value1",
+			},
+			input: map[string]interface{}{},
+			expected: map[string]interface{}{
+				"key1": "",
+			},
+		},
+		{
+			name:  "MapUnsetWithNilNotSet",
+			state: map[string]interface{}{},
+			input: map[string]interface{}{
+				"key1": nil,
+			},
+			expected: map[string]interface{}{
+				"key1": "",
+			},
+		},
+		{
+			name:  "MapUnsetWithEmptyNotSet",
+			state: map[string]interface{}{},
+			input: map[string]interface{}{
+				"key1": "",
+			},
+			expected: map[string]interface{}{
+				"key1": "",
+			},
+		},
+		{
+			name:  "MapSetNew",
+			state: map[string]interface{}{},
+			input: map[string]interface{}{
+				"key1": "test",
+			},
+			expected: map[string]interface{}{
+				"key1": "test",
+			},
+		},
+		{
+			name: "MapSetExisting",
+			state: map[string]interface{}{
+				"key1": " test",
+			},
+			input: map[string]interface{}{
+				"key1": "testnew",
+			},
+			expected: map[string]interface{}{
+				"key1": "testnew",
+			},
+		},
+		{
+			name: "MapMixed",
+			state: map[string]interface{}{
+				"key1": "value1",
+				"key2": "value2",
+				"key3": "",
+				"key4": nil,
+			},
+			input: map[string]interface{}{
+				"key1":   "valuenew1",
+				"key2":   nil,
+				"keynew": "valuenew2",
+			},
+			expected: map[string]interface{}{
+				"key1":   "valuenew1",
+				"key2":   "",
+				"key3":   "",
+				"key4":   "",
+				"keynew": "valuenew2",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := mergeCustomFields(tt.state, tt.input)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}