diff --git a/Tests/test_file_container.py b/Tests/test_file_container.py
index 6abf5cc1451..ad0394cf9d4 100644
--- a/Tests/test_file_container.py
+++ b/Tests/test_file_container.py
@@ -57,69 +57,83 @@ def test_seek_mode_2(self):
 
     def test_read_n0(self):
         # Arrange
-        with open(TEST_FILE, "rb") as fh:
-            container = ContainerIO.ContainerIO(fh, 22, 100)
+        for bytesmode in (True, False):
+            with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+                container = ContainerIO.ContainerIO(fh, 22, 100)
 
-            # Act
-            container.seek(81)
-            data = container.read()
+                # Act
+                container.seek(81)
+                data = container.read()
 
-            # Assert
-            self.assertEqual(data, b"7\nThis is line 8\n")
+                # Assert
+                if bytesmode:
+                    data = data.decode()
+                self.assertEqual(data, "7\nThis is line 8\n")
 
     def test_read_n(self):
         # Arrange
-        with open(TEST_FILE, "rb") as fh:
-            container = ContainerIO.ContainerIO(fh, 22, 100)
+        for bytesmode in (True, False):
+            with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+                container = ContainerIO.ContainerIO(fh, 22, 100)
 
-            # Act
-            container.seek(81)
-            data = container.read(3)
+                # Act
+                container.seek(81)
+                data = container.read(3)
 
-            # Assert
-            self.assertEqual(data, b"7\nT")
+                # Assert
+                if bytesmode:
+                    data = data.decode()
+                self.assertEqual(data, "7\nT")
 
     def test_read_eof(self):
         # Arrange
-        with open(TEST_FILE, "rb") as fh:
-            container = ContainerIO.ContainerIO(fh, 22, 100)
+        for bytesmode in (True, False):
+            with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+                container = ContainerIO.ContainerIO(fh, 22, 100)
 
-            # Act
-            container.seek(100)
-            data = container.read()
+                # Act
+                container.seek(100)
+                data = container.read()
 
-            # Assert
-            self.assertEqual(data, b"")
+                # Assert
+                if bytesmode:
+                    data = data.decode()
+                self.assertEqual(data, "")
 
     def test_readline(self):
         # Arrange
-        with open(TEST_FILE, "rb") as fh:
-            container = ContainerIO.ContainerIO(fh, 0, 120)
+        for bytesmode in (True, False):
+            with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+                container = ContainerIO.ContainerIO(fh, 0, 120)
 
-            # Act
-            data = container.readline()
+                # Act
+                data = container.readline()
 
-            # Assert
-            self.assertEqual(data, b"This is line 1\n")
+                # Assert
+                if bytesmode:
+                    data = data.decode()
+                self.assertEqual(data, "This is line 1\n")
 
     def test_readlines(self):
         # Arrange
-        expected = [
-            b"This is line 1\n",
-            b"This is line 2\n",
-            b"This is line 3\n",
-            b"This is line 4\n",
-            b"This is line 5\n",
-            b"This is line 6\n",
-            b"This is line 7\n",
-            b"This is line 8\n",
-        ]
-        with open(TEST_FILE, "rb") as fh:
-            container = ContainerIO.ContainerIO(fh, 0, 120)
-
-            # Act
-            data = container.readlines()
-
-            # Assert
-
-            self.assertEqual(data, expected)
+        for bytesmode in (True, False):
+            expected = [
+                "This is line 1\n",
+                "This is line 2\n",
+                "This is line 3\n",
+                "This is line 4\n",
+                "This is line 5\n",
+                "This is line 6\n",
+                "This is line 7\n",
+                "This is line 8\n",
+            ]
+            with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
+                container = ContainerIO.ContainerIO(fh, 0, 120)
+
+                # Act
+                data = container.readlines()
+
+                # Assert
+                if bytesmode:
+                    data = [line.decode() for line in data]
+                self.assertEqual(data, expected)
diff --git a/src/PIL/ContainerIO.py b/src/PIL/ContainerIO.py
index b8a3d0582dd..10ac6735d56 100644
--- a/src/PIL/ContainerIO.py
+++ b/src/PIL/ContainerIO.py
@@ -82,7 +82,7 @@ def read(self, n=0):
         else:
             n = self.length - self.pos
         if not n:  # EOF
-            return b""
+            return b"" if "b" in self.fh.mode else ""
         self.pos = self.pos + n
         return self.fh.read(n)
 
@@ -92,13 +92,13 @@ def readline(self):
 
         :returns: An 8-bit string.
         """
-        s = b""
+        s = b"" if "b" in self.fh.mode else ""
         while True:
             c = self.read(1)
             if not c:
                 break
             s = s + c
-            if c == b"\n":
+            if c == (b"\n" if "b" in self.fh.mode else "\n"):
                 break
         return s