diff --git a/server/events/comment_parser.go b/server/events/comment_parser.go
index 7445cda96..198439e71 100644
--- a/server/events/comment_parser.go
+++ b/server/events/comment_parser.go
@@ -16,6 +16,7 @@ package events
 import (
 	"fmt"
 	"io/ioutil"
+	"net/url"
 	"path/filepath"
 	"strings"
 
@@ -189,10 +190,11 @@ func (e *CommentParser) Parse(comment string, vcsHost vcs.Host) CommentParseResu
 		return CommentParseResult{CommentResponse: e.errMarkdown(err.Error(), command, flagSet)}
 	}
 
-	// Because we use the workspace name as a file, need to make sure it's
-	// not doing something weird like being a relative dir.
-	if strings.Contains(workspace, "..") {
-		return CommentParseResult{CommentResponse: e.errMarkdown(fmt.Sprintf("value for -%s/--%s can't contain '..'", WorkspaceFlagShort, WorkspaceFlagLong), command, flagSet)}
+	// Use the same validation that Terraform uses: https://git.io/vxGhU. Plus
+	// we also don't allow '..'. We don't want the workspace to contain a path
+	// since we create files based on the name.
+	if workspace != url.PathEscape(workspace) || strings.Contains(workspace, "..") {
+		return CommentParseResult{CommentResponse: e.errMarkdown(fmt.Sprintf("invalid workspace: %q", workspace), command, flagSet)}
 	}
 
 	return CommentParseResult{
diff --git a/server/events/comment_parser_test.go b/server/events/comment_parser_test.go
index 41b556048..8edbc1417 100644
--- a/server/events/comment_parser_test.go
+++ b/server/events/comment_parser_test.go
@@ -240,10 +240,12 @@ func TestParse_RelativeDirPath(t *testing.T) {
 }
 
 func TestParse_InvalidWorkspace(t *testing.T) {
-	t.Log("if -w is used with '..', should return an error")
+	t.Log("if -w is used with '..' or '/', should return an error")
 	comments := []string{
 		"atlantis plan -w ..",
 		"atlantis apply -w ..",
+		"atlantis plan -w /",
+		"atlantis apply -w /",
 		"atlantis plan -w ..abc",
 		"atlantis apply -w abc..",
 		"atlantis plan -w abc..abc",
@@ -251,7 +253,7 @@ func TestParse_InvalidWorkspace(t *testing.T) {
 	}
 	for _, c := range comments {
 		r := commentParser.Parse(c, vcs.Github)
-		exp := "Error: value for -w/--workspace can't contain '..'"
+		exp := "Error: invalid workspace"
 		Assert(t, strings.Contains(r.CommentResponse, exp),
 			"For comment %q expected CommentResponse %q to contain %q", c, r.CommentResponse, exp)
 	}