diff --git a/helper/schema/field_reader_config.go b/helper/schema/field_reader_config.go
index 808375ceb..6ad3f13cb 100644
--- a/helper/schema/field_reader_config.go
+++ b/helper/schema/field_reader_config.go
@@ -219,6 +219,9 @@ func (r *ConfigFieldReader) readMap(k string, schema *Schema) (FieldReadResult,
 			v, _ := r.Config.Get(key)
 			result[ik] = v
 		}
+	case nil:
+		// the map may have been empty on the configuration, so we leave the
+		// empty result
 	default:
 		panic(fmt.Sprintf("unknown type: %#v", mraw))
 	}
diff --git a/helper/schema/field_reader_diff.go b/helper/schema/field_reader_diff.go
index ae35b4a87..3e70acf0b 100644
--- a/helper/schema/field_reader_diff.go
+++ b/helper/schema/field_reader_diff.go
@@ -95,7 +95,9 @@ func (r *DiffFieldReader) readMap(
 		return FieldReadResult{}, err
 	}
 	if source.Exists {
-		result = source.Value.(map[string]interface{})
+		// readMap may return a nil value, or an unknown value placeholder in
+		// some cases, causing the type assertion to panic if we don't assign the ok value
+		result, _ = source.Value.(map[string]interface{})
 		resultSet = true
 	}