diff --git a/protogen/names.go b/protogen/names.go
index e291805..a1bddf7 100644
--- a/protogen/names.go
+++ b/protogen/names.go
@@ -83,48 +83,6 @@
 	return s
 }
 
-var isGoPredeclaredIdentifier = map[string]bool{
-	"append":     true,
-	"bool":       true,
-	"byte":       true,
-	"cap":        true,
-	"close":      true,
-	"complex":    true,
-	"complex128": true,
-	"complex64":  true,
-	"copy":       true,
-	"delete":     true,
-	"error":      true,
-	"false":      true,
-	"float32":    true,
-	"float64":    true,
-	"imag":       true,
-	"int":        true,
-	"int16":      true,
-	"int32":      true,
-	"int64":      true,
-	"int8":       true,
-	"iota":       true,
-	"len":        true,
-	"make":       true,
-	"new":        true,
-	"nil":        true,
-	"panic":      true,
-	"print":      true,
-	"println":    true,
-	"real":       true,
-	"recover":    true,
-	"rune":       true,
-	"string":     true,
-	"true":       true,
-	"uint":       true,
-	"uint16":     true,
-	"uint32":     true,
-	"uint64":     true,
-	"uint8":      true,
-	"uintptr":    true,
-}
-
 // baseName returns the last path element of the name, with the last dotted suffix removed.
 func baseName(name string) string {
 	// First, find the last element
diff --git a/protogen/protogen.go b/protogen/protogen.go
index 9c279b8..eb89cfa 100644
--- a/protogen/protogen.go
+++ b/protogen/protogen.go
@@ -19,6 +19,7 @@
 	"go/parser"
 	"go/printer"
 	"go/token"
+	"go/types"
 	"io/ioutil"
 	"os"
 	"path"
@@ -851,6 +852,12 @@
 		manualImports:    make(map[GoImportPath]bool),
 		annotations:      make(map[string][]Location),
 	}
+
+	// All predeclared identifiers in Go are already used.
+	for _, s := range types.Universe.Names() {
+		g.usedPackageNames[GoPackageName(s)] = true
+	}
+
 	gen.genFiles = append(gen.genFiles, g)
 	return g
 }
@@ -906,7 +913,7 @@
 		return string(packageName) + "." + ident.GoName
 	}
 	packageName := cleanPackageName(baseName(string(ident.GoImportPath)))
-	for i, orig := 1, packageName; g.usedPackageNames[packageName] || isGoPredeclaredIdentifier[string(packageName)]; i++ {
+	for i, orig := 1, packageName; g.usedPackageNames[packageName]; i++ {
 		packageName = orig + GoPackageName(strconv.Itoa(i))
 	}
 	g.packageNames[ident.GoImportPath] = packageName