diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 2aa6f61..774b29c 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -12,7 +12,7 @@ jobs:
   ci:
     strategy:
       matrix:
-        go: ['1.15', '1.14', '1.13']
+        go: ['1.16', '1.17']
         platform: [ubuntu-latest, macos-latest] # can not run in windows OS
     runs-on: ${{ matrix.platform }}
 
diff --git a/copier.go b/copier.go
index 246bfed..6d21da8 100644
--- a/copier.go
+++ b/copier.go
@@ -348,7 +348,10 @@ func deepFields(reflectType reflect.Type) []reflect.StructField {
 
 		for i := 0; i < reflectType.NumField(); i++ {
 			v := reflectType.Field(i)
-			if v.IsExported() {
+			// PkgPath is the package path that qualifies a lower case (unexported)
+			// field name. It is empty for upper case (exported) field names.
+			// See https://golang.org/ref/spec#Uniqueness_of_identifiers
+			if v.PkgPath == "" {
 				if v.Anonymous {
 					fields = append(fields, deepFields(v.Type)...)
 				} else {
diff --git a/go.mod b/go.mod
index 531422d..309801e 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/jinzhu/copier
 
-go 1.15
+go 1.13