diff --git a/internal/provider/resource_password.go b/internal/provider/resource_password.go
index c14a8f71..aa82c7de 100644
--- a/internal/provider/resource_password.go
+++ b/internal/provider/resource_password.go
@@ -11,10 +11,14 @@ import (
 )
 
 // resourcePassword and resourceString both use the same set of CustomizeDiffFunc(s) in order to handle the deprecation
-// of the `number` attribute and the simultaneous addition of the `numeric` attribute. customDiffIfValue handles
+// of the `number` attribute and the simultaneous addition of the `numeric` attribute. planDefaultIfAllNull handles
 // ensuring that both `number` and `numeric` default to `true` when they are both absent from config.
-// customDiffIfValueChange handles keeping number and numeric in-sync when either one has been changed.
+// planSyncIfChange handles keeping number and numeric in-sync when either one has been changed.
 func resourcePassword() *schema.Resource {
+	customizeDiffFuncs := planDefaultIfAllNull(true, "number", "numeric")
+	customizeDiffFuncs = append(customizeDiffFuncs, planSyncIfChange("number", "numeric"))
+	customizeDiffFuncs = append(customizeDiffFuncs, planSyncIfChange("numeric", "number"))
+
 	return &schema.Resource{
 		Description: "Identical to [random_string](string.html) with the exception that the result is " +
 			"treated as sensitive and, thus, _not_ displayed in console output. Read more about sensitive " +
@@ -42,10 +46,7 @@ func resourcePassword() *schema.Resource {
 			},
 		},
 		CustomizeDiff: customdiff.All(
-			customDiffIfValue("number"),
-			customDiffIfValue("numeric"),
-			customDiffIfValueChange("number", "numeric"),
-			customDiffIfValueChange("numeric", "number"),
+			customizeDiffFuncs...,
 		),
 	}
 }
diff --git a/internal/provider/resource_string.go b/internal/provider/resource_string.go
index b0962133..5749aeb0 100644
--- a/internal/provider/resource_string.go
+++ b/internal/provider/resource_string.go
@@ -9,10 +9,14 @@ import (
 )
 
 // resourceString and resourcePassword both use the same set of CustomizeDiffFunc(s) in order to handle the deprecation
-// of the `number` attribute and the simultaneous addition of the `numeric` attribute. customDiffIfValue handles
+// of the `number` attribute and the simultaneous addition of the `numeric` attribute. planDefaultIfAllNull handles
 // ensuring that both `number` and `numeric` default to `true` when they are both absent from config.
-// customDiffIfValueChange handles keeping number and numeric in-sync when either one has been changed.
+// planSyncIfChange handles keeping number and numeric in-sync when either one has been changed.
 func resourceString() *schema.Resource {
+	customizeDiffFuncs := planDefaultIfAllNull(true, "number", "numeric")
+	customizeDiffFuncs = append(customizeDiffFuncs, planSyncIfChange("number", "numeric"))
+	customizeDiffFuncs = append(customizeDiffFuncs, planSyncIfChange("numeric", "number"))
+
 	return &schema.Resource{
 		Description: "The resource `random_string` generates a random permutation of alphanumeric " +
 			"characters and optionally special characters.\n" +
@@ -41,10 +45,7 @@ func resourceString() *schema.Resource {
 			},
 		},
 		CustomizeDiff: customdiff.All(
-			customDiffIfValue("number"),
-			customDiffIfValue("numeric"),
-			customDiffIfValueChange("number", "numeric"),
-			customDiffIfValueChange("numeric", "number"),
+			customizeDiffFuncs...,
 		),
 	}
 }
diff --git a/internal/provider/string.go b/internal/provider/string.go
index 072b25a7..3e2eb863 100644
--- a/internal/provider/string.go
+++ b/internal/provider/string.go
@@ -6,6 +6,7 @@ package provider
 import (
 	"context"
 	"crypto/rand"
+	"errors"
 	"fmt"
 	"math/big"
 	"sort"
@@ -340,36 +341,53 @@ func resourceStateUpgradeAddNumeric(resourceName string) func(_ context.Context,
 	}
 }
 
-// customDiffIfValue handles ensuring that both `number` and `numeric` attributes default to `true` when neither are set
+// planDefaultIfAllNull handles ensuring that both `number` and `numeric` attributes default to `true` when neither are set
 // in the config and, they had been previously set to `false`. This behaviour mimics setting `Default: true` on the
 // attributes. Usage of `Default` is avoided as `Default` cannot be used with CustomizeDiffFunc(s) which are required in
-// order to keep `number` and `numeric` in-sync (see customDiffIfValueChange).
-func customDiffIfValue(key string) func(context.Context, *schema.ResourceDiff, interface{}) error {
-	return customdiff.IfValue(
-		key,
-		func(ctx context.Context, value, meta interface{}) bool {
-			return !value.(bool)
-		},
-		func(_ context.Context, d *schema.ResourceDiff, _ interface{}) error {
-			vm := d.GetRawConfig().AsValueMap()
-			if vm["number"].IsNull() && vm["numeric"].IsNull() {
-				err := d.SetNew("number", true)
-				if err != nil {
-					return err
+// order to keep `number` and `numeric` in-sync (see planSyncIfChange).
+func planDefaultIfAllNull(defaultVal interface{}, keys ...string) []schema.CustomizeDiffFunc {
+	var result []schema.CustomizeDiffFunc
+
+	for _, key := range keys {
+		result = append(result, customdiff.IfValue(
+			key,
+			func(ctx context.Context, value, meta interface{}) bool {
+				return !value.(bool)
+			},
+			func(_ context.Context, d *schema.ResourceDiff, _ interface{}) error {
+				vm := d.GetRawConfig().AsValueMap()
+
+				number, ok := vm["number"]
+				if !ok {
+					return errors.New("number is absent from raw config")
 				}
-				err = d.SetNew("numeric", true)
-				if err != nil {
-					return err
+
+				numeric, ok := vm["numeric"]
+				if !ok {
+					return errors.New("numeric is absent from raw config")
 				}
-			}
-			return nil
-		},
-	)
+
+				if number.IsNull() && numeric.IsNull() {
+					err := d.SetNew("number", defaultVal)
+					if err != nil {
+						return err
+					}
+					err = d.SetNew("numeric", defaultVal)
+					if err != nil {
+						return err
+					}
+				}
+				return nil
+			},
+		))
+	}
+
+	return result
 }
 
-// customDiffIfValueChange handles keeping `number` and `numeric` in-sync. If either is changed the value of both is
+// planSyncIfChange handles keeping `number` and `numeric` in-sync. If either is changed the value of both is
 // set to the new value of the attribute that has changed.
-func customDiffIfValueChange(key, keyToSync string) func(context.Context, *schema.ResourceDiff, interface{}) error {
+func planSyncIfChange(key, keyToSync string) func(context.Context, *schema.ResourceDiff, interface{}) error {
 	return customdiff.IfValueChange(
 		key,
 		func(ctx context.Context, oldValue, newValue, meta interface{}) bool {