diff --git a/internal/command/providers_lock.go b/internal/command/providers_lock.go
index ba61146a4..a7243fab5 100644
--- a/internal/command/providers_lock.go
+++ b/internal/command/providers_lock.go
@@ -128,8 +128,8 @@ func (c *ProvidersLockCommand) Run(args []string) int {
 				// current configuration.
 				diags = diags.Append(tfdiags.Sourceless(
 					tfdiags.Error,
-					"Invalid network mirror URL",
-					"The -net-mirror option requires a valid https: URL as the mirror base URL.",
+					"Invalid provider argument",
+					fmt.Sprintf("The provider %s is not required by the current configuration.", addr.String()),
 				))
 			}
 		}
diff --git a/internal/command/providers_lock_test.go b/internal/command/providers_lock_test.go
new file mode 100644
index 000000000..da6cca6ec
--- /dev/null
+++ b/internal/command/providers_lock_test.go
@@ -0,0 +1,155 @@
+package command
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+
+	"github.com/mitchellh/cli"
+)
+
+func TestProvidersLock(t *testing.T) {
+	t.Run("noop", func(t *testing.T) {
+		// in the most basic case, running providers lock in a directory with no configuration at all should succeed.
+		// create an empty working directory
+		td := tempDir(t)
+		os.MkdirAll(td, 0755)
+		defer os.RemoveAll(td)
+		defer testChdir(t, td)()
+
+		ui := new(cli.MockUi)
+		c := &ProvidersLockCommand{
+			Meta: Meta{
+				Ui: ui,
+			},
+		}
+		code := c.Run([]string{})
+		if code != 0 {
+			t.Fatalf("wrong exit code; expected 0, got %d", code)
+		}
+	})
+
+	// This test depends on the -fs-mirror argument, so we always know what results to expect
+	t.Run("basic", func(t *testing.T) {
+		td := tempDir(t)
+		testCopyDir(t, testFixturePath("providers-lock/basic"), td)
+		defer os.RemoveAll(td)
+		defer testChdir(t, td)()
+
+		// Our fixture dir has a generic os_arch dir, which we need to customize
+		// to the actual OS/arch where this test is running in order to get the
+		// desired result.
+		fixtMachineDir := filepath.Join(td, "fs-mirror/registry.terraform.io/hashicorp/test/1.0.0/os_arch")
+		wantMachineDir := filepath.Join(td, "fs-mirror/registry.terraform.io/hashicorp/test/1.0.0/", fmt.Sprintf("%s_%s", runtime.GOOS, runtime.GOARCH))
+		err := os.Rename(fixtMachineDir, wantMachineDir)
+		if err != nil {
+			t.Fatalf("unexpected error: %s", err)
+		}
+
+		p := testProvider()
+		ui := new(cli.MockUi)
+		c := &ProvidersLockCommand{
+			Meta: Meta{
+				Ui:               ui,
+				testingOverrides: metaOverridesForProvider(p),
+			},
+		}
+
+		args := []string{"-fs-mirror=fs-mirror"}
+		code := c.Run(args)
+		if code != 0 {
+			t.Fatalf("wrong exit code; expected 0, got %d", code)
+		}
+
+		lockfile, err := os.ReadFile(".terraform.lock.hcl")
+		if err != nil {
+			t.Fatal("error reading lockfile")
+		}
+
+		expected := `# This file is maintained automatically by "terraform init".
+# Manual edits may be lost in future updates.
+
+provider "registry.terraform.io/hashicorp/test" {
+  version = "1.0.0"
+  hashes = [
+    "h1:7MjN4eFisdTv4tlhXH5hL4QQd39Jy4baPhFxwAd/EFE=",
+  ]
+}
+`
+		if string(lockfile) != expected {
+			t.Fatalf("wrong lockfile content")
+		}
+	})
+}
+
+func TestProvidersLock_args(t *testing.T) {
+
+	t.Run("mirror collision", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &ProvidersLockCommand{
+			Meta: Meta{
+				Ui: ui,
+			},
+		}
+
+		// only one of these arguments can be used at a time
+		args := []string{
+			"-fs-mirror=/foo/",
+			"-net-mirror=www.foo.com",
+		}
+		code := c.Run(args)
+
+		if code != 1 {
+			t.Fatalf("wrong exit code; expected 1, got %d", code)
+		}
+		output := ui.ErrorWriter.String()
+		if !strings.Contains(output, "The -fs-mirror and -net-mirror command line options are mutually-exclusive.") {
+			t.Fatalf("missing expected error message: %s", output)
+		}
+	})
+
+	t.Run("invalid platform", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &ProvidersLockCommand{
+			Meta: Meta{
+				Ui: ui,
+			},
+		}
+
+		// not a valid platform
+		args := []string{"-platform=arbitrary_nonsense_that_isnt_valid"}
+		code := c.Run(args)
+
+		if code != 1 {
+			t.Fatalf("wrong exit code; expected 1, got %d", code)
+		}
+		output := ui.ErrorWriter.String()
+		if !strings.Contains(output, "must be two words separated by an underscore.") {
+			t.Fatalf("missing expected error message: %s", output)
+		}
+	})
+
+	t.Run("invalid provider argument", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &ProvidersLockCommand{
+			Meta: Meta{
+				Ui: ui,
+			},
+		}
+
+		// There is no configuration, so it's not valid to use any provider argument
+		args := []string{"hashicorp/random"}
+		code := c.Run(args)
+
+		if code != 1 {
+			t.Fatalf("wrong exit code; expected 1, got %d", code)
+		}
+		output := ui.ErrorWriter.String()
+		if !strings.Contains(output, "The provider registry.terraform.io/hashicorp/random is not required by the\ncurrent configuration.") {
+			t.Fatalf("missing expected error message: %s", output)
+		}
+	})
+}
diff --git a/internal/command/providers_mirror_test.go b/internal/command/providers_mirror_test.go
new file mode 100644
index 000000000..073dd6f30
--- /dev/null
+++ b/internal/command/providers_mirror_test.go
@@ -0,0 +1,36 @@
+package command
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/mitchellh/cli"
+)
+
+// More thorough tests for providers mirror can be found in the e2etest
+func TestProvidersMirror(t *testing.T) {
+	// noop example
+	t.Run("noop", func(t *testing.T) {
+		c := &ProvidersMirrorCommand{}
+		code := c.Run([]string{"."})
+		if code != 0 {
+			t.Fatalf("wrong exit code. expected 0, got %d", code)
+		}
+	})
+
+	t.Run("missing arg error", func(t *testing.T) {
+		ui := new(cli.MockUi)
+		c := &ProvidersMirrorCommand{
+			Meta: Meta{Ui: ui},
+		}
+		code := c.Run([]string{})
+		if code != 1 {
+			t.Fatalf("wrong exit code. expected 1, got %d", code)
+		}
+
+		got := ui.ErrorWriter.String()
+		if !strings.Contains(got, "Error: No output directory specified") {
+			t.Fatalf("missing directory error from output, got:\n%s\n", got)
+		}
+	})
+}
diff --git a/internal/command/testdata/providers-lock/basic/fs-mirror/registry.terraform.io/hashicorp/test/1.0.0/os_arch/terraform-provider-test b/internal/command/testdata/providers-lock/basic/fs-mirror/registry.terraform.io/hashicorp/test/1.0.0/os_arch/terraform-provider-test
new file mode 100644
index 000000000..e69de29bb
diff --git a/internal/command/testdata/providers-lock/basic/main.tf b/internal/command/testdata/providers-lock/basic/main.tf
new file mode 100644
index 000000000..41b211f26
--- /dev/null
+++ b/internal/command/testdata/providers-lock/basic/main.tf
@@ -0,0 +1,7 @@
+terraform {
+    required_providers {
+        test = {
+            source = "hashicorp/test"
+        }
+    }
+}
\ No newline at end of file
diff --git a/internal/getproviders/types.go b/internal/getproviders/types.go
index a6498386b..28b1913d6 100644
--- a/internal/getproviders/types.go
+++ b/internal/getproviders/types.go
@@ -137,12 +137,12 @@ func (p Platform) LessThan(other Platform) bool {
 // ParsePlatform parses a string representation of a platform, like
 // "linux_amd64", or returns an error if the string is not valid.
 func ParsePlatform(str string) (Platform, error) {
-	underPos := strings.Index(str, "_")
-	if underPos < 1 || underPos >= len(str)-2 {
+	parts := strings.Split(str, "_")
+	if len(parts) != 2 {
 		return Platform{}, fmt.Errorf("must be two words separated by an underscore")
 	}
 
-	os, arch := str[:underPos], str[underPos+1:]
+	os, arch := parts[0], parts[1]
 	if strings.ContainsAny(os, " \t\n\r") {
 		return Platform{}, fmt.Errorf("OS portion must not contain whitespace")
 	}
diff --git a/internal/getproviders/types_test.go b/internal/getproviders/types_test.go
index b12cc2155..0008793a1 100644
--- a/internal/getproviders/types_test.go
+++ b/internal/getproviders/types_test.go
@@ -94,3 +94,48 @@ func TestVersionConstraintsString(t *testing.T) {
 		})
 	}
 }
+
+func TestParsePlatform(t *testing.T) {
+	tests := []struct {
+		Input string
+		Want  Platform
+		Err   bool
+	}{
+		{
+			"",
+			Platform{},
+			true,
+		},
+		{
+			"too_many_notes",
+			Platform{},
+			true,
+		},
+		{
+			"extra _ whitespaces ",
+			Platform{},
+			true,
+		},
+		{
+			"arbitrary_os",
+			Platform{OS: "arbitrary", Arch: "os"},
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		got, err := ParsePlatform(test.Input)
+		if err != nil {
+			if test.Err == false {
+				t.Errorf("unexpected error: %s", err.Error())
+			}
+		} else {
+			if test.Err {
+				t.Errorf("wrong result: expected error, got none")
+			}
+		}
+		if got != test.Want {
+			t.Errorf("wrong\n got: %q\nwant: %q", got, test.Want)
+		}
+	}
+}