diff --git a/graalpython/com.oracle.graal.python.test/src/tests/test_io.py b/graalpython/com.oracle.graal.python.test/src/tests/test_io.py
index 2bad5600b3..be3a9f2edc 100644
--- a/graalpython/com.oracle.graal.python.test/src/tests/test_io.py
+++ b/graalpython/com.oracle.graal.python.test/src/tests/test_io.py
@@ -353,6 +353,12 @@ def test_stringio_overwrite(self):
         s.write('ab')
         self.assertEqual('heabo', s.getvalue())
 
+    def test_stringio_validates_newline_before_initial_value(self):
+        self.assertRaises(ValueError, _io.StringIO, b'', 'x')
+        self.assertRaises(ValueError, _io.StringIO, 1, 'x')
+        self.assertRaises(TypeError, _io.StringIO, b'')
+        self.assertEqual('', _io.StringIO(None).getvalue())
+
     def test_cr_not_ignored(self):
         d = _io.IncrementalNewlineDecoder(None, translate=False)
         d.decode("h\rello")
diff --git a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/io/StringIOBuiltins.java b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/io/StringIOBuiltins.java
index 84b2bc626f..2b265b27a8 100644
--- a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/io/StringIOBuiltins.java
+++ b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/io/StringIOBuiltins.java
@@ -61,6 +61,7 @@
 import static com.oracle.graal.python.builtins.modules.io.IONodes.T_READLINE;
 import static com.oracle.graal.python.builtins.modules.io.TextIOWrapperNodes.validateNewline;
 import static com.oracle.graal.python.nodes.ErrorMessages.CAN_T_DO_NONZERO_CUR_RELATIVE_SEEKS;
+import static com.oracle.graal.python.nodes.ErrorMessages.INITIAL_VALUE_MUST_BE_STR_OR_NONE_NOT_P;
 import static com.oracle.graal.python.nodes.ErrorMessages.INVALID_WHENCE_D_SHOULD_BE_0_1_OR_2;
 import static com.oracle.graal.python.nodes.ErrorMessages.IO_CLOSED;
 import static com.oracle.graal.python.nodes.ErrorMessages.IO_UNINIT;
@@ -116,11 +117,13 @@
 import com.oracle.graal.python.nodes.function.PythonBuiltinNode;
 import com.oracle.graal.python.nodes.function.builtins.PythonBinaryBuiltinNode;
 import com.oracle.graal.python.nodes.function.builtins.PythonBinaryClinicBuiltinNode;
+import com.oracle.graal.python.nodes.function.builtins.PythonTernaryBuiltinNode;
 import com.oracle.graal.python.nodes.function.builtins.PythonTernaryClinicBuiltinNode;
 import com.oracle.graal.python.nodes.function.builtins.PythonUnaryBuiltinNode;
 import com.oracle.graal.python.nodes.function.builtins.clinic.ArgumentClinicProvider;
 import com.oracle.graal.python.nodes.object.BuiltinClassProfiles.IsBuiltinObjectExactProfile;
 import com.oracle.graal.python.nodes.object.GetOrCreateDictNode;
+import com.oracle.graal.python.nodes.util.CannotCastException;
 import com.oracle.graal.python.nodes.util.CastToTruffleStringNode;
 import com.oracle.graal.python.runtime.object.PFactory;
 import com.oracle.graal.python.runtime.sequence.storage.SequenceStorage;
@@ -280,17 +283,11 @@ static PStringIO doNew(Object cls, @SuppressWarnings("unused") Object arg,
 
     @Slot(value = SlotKind.tp_init, isComplex = true)
     @SlotSignature(name = "StringIO", minNumOfPositionalArgs = 1, parameterNames = {"$self", "initial_value", "newline"})
-    @ArgumentClinic(name = "initial_value", conversion = ArgumentClinic.ClinicConversion.TString, defaultValue = "T_EMPTY_STRING", useDefaultForNone = true)
     @GenerateNodeFactory
-    public abstract static class InitNode extends PythonTernaryClinicBuiltinNode {
-
-        @Override
-        protected ArgumentClinicProvider getArgumentClinic() {
-            return StringIOBuiltinsClinicProviders.InitNodeClinicProviderGen.INSTANCE;
-        }
+    public abstract static class InitNode extends PythonTernaryBuiltinNode {
 
         @Specialization
-        static PNone init(VirtualFrame frame, PStringIO self, TruffleString initialValue, Object newlineArg,
+        static PNone init(VirtualFrame frame, PStringIO self, Object initialValueArg, Object newlineArg,
                         @Bind Node inliningTarget,
                         @Cached PRaiseNode lazyRaiseNode,
                         @Cached IncrementalNewlineDecoderBuiltins.DecodeNode decodeNode,
@@ -303,6 +300,7 @@ static PNone init(VirtualFrame frame, PStringIO self, TruffleString initialValue
                         @Cached TruffleStringBuilder.ToStringNode toStringNode,
                         @Cached TruffleStringBuilder.AppendCodePointNode appendCodePointNode,
                         @Cached IONodes.ToTruffleStringNode toTruffleStringNode,
+                        @Cached CastToTruffleStringNode castToStringNode,
                         @Bind PythonLanguage language,
                         @Cached PRaiseNode raiseNode) {
             TruffleString newline;
@@ -318,6 +316,16 @@ static PNone init(VirtualFrame frame, PStringIO self, TruffleString initialValue
             if (newline != null) {
                 validateNewline(newline, inliningTarget, lazyRaiseNode, codePointLengthNode, codePointAtIndexNode);
             }
+            TruffleString initialValue;
+            if (PGuards.isPNone(initialValueArg)) {
+                initialValue = T_EMPTY_STRING;
+            } else {
+                try {
+                    initialValue = castToStringNode.execute(inliningTarget, initialValueArg);
+                } catch (CannotCastException e) {
+                    throw raiseNode.raise(inliningTarget, TypeError, INITIAL_VALUE_MUST_BE_STR_OR_NONE_NOT_P, initialValueArg);
+                }
+            }
             self.setOK(false);
             self.clearAll();
 
diff --git a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java
index 410f9a9f28..1abe2a5749 100644
--- a/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java
+++ b/graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java
@@ -999,6 +999,7 @@ public abstract class ErrorMessages {
     public static final TruffleString INPUT_FMT_NOT_SUPPORTED_BY_DECODER = tsLiteral("Input format not supported by decoder");
     public static final TruffleString INVALID_UNSUPPORTED_OPTIONS = tsLiteral("Invalid or unsupported options");
     public static final TruffleString INDENT_MUST_BE_STR_OR_NONE_NOT_P = tsLiteral("indent must be str or None, not %p");
+    public static final TruffleString INITIAL_VALUE_MUST_BE_STR_OR_NONE_NOT_P = tsLiteral("initial_value must be str or None, not %p");
     public static final TruffleString CORRUPT_INPUT_DATA = tsLiteral("Corrupt input data");
     public static final TruffleString INSUFFICIENT_BUFFER_SPACE = tsLiteral("Insufficient buffer space");
     public static final TruffleString INTERNAL_ERROR = tsLiteral("Internal error");