diff --git a/CHANGELOG.md b/CHANGELOG.md
index dbdb990a6..f68ff7c92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 # Unreleased
 
+## Fixed
+* Erroring on constructing an odd length hex string. ([#539](https://github.com/algorand/pyteal/pull/539))
+
 # 0.18.1
 
 ## Fixed
diff --git a/pyteal/ast/bytes.py b/pyteal/ast/bytes.py
index 4c4fb4d6e..1c7bd09b4 100644
--- a/pyteal/ast/bytes.py
+++ b/pyteal/ast/bytes.py
@@ -1,4 +1,4 @@
-from typing import Union, cast, overload, TYPE_CHECKING
+from typing import cast, overload, TYPE_CHECKING
 
 from pyteal.types import TealType, valid_base16, valid_base32, valid_base64
 from pyteal.util import escapeStr
@@ -14,14 +14,14 @@ class Bytes(LeafExpr):
     """An expression that represents a byte string."""
 
     @overload
-    def __init__(self, arg1: Union[str, bytes, bytearray]) -> None:
+    def __init__(self, arg1: str | bytes | bytearray) -> None:
         pass
 
     @overload
     def __init__(self, arg1: str, arg2: str) -> None:
         pass
 
-    def __init__(self, arg1: Union[str, bytes, bytearray], arg2: str = None) -> None:
+    def __init__(self, arg1: str | bytes | bytearray, arg2: str = None) -> None:
         """
         __init__(arg1: Union[str, bytes, bytearray]) -> None
         __init__(self, arg1: str, arg2: str) -> None
@@ -49,15 +49,15 @@ def __init__(self, arg1: Union[str, bytes, bytearray], arg2: str = None) -> None
                 self.byte_str = escapeStr(arg1)
             elif type(arg1) in (bytes, bytearray):
                 self.base = "base16"
-                self.byte_str = cast(Union[bytes, bytearray], arg1).hex()
+                self.byte_str = cast(bytes | bytearray, arg1).hex()
             else:
-                raise TealInputError("Unknown argument type: {}".format(type(arg1)))
+                raise TealInputError(f"Unknown argument type: {type(arg1)}")
         else:
             if type(arg1) is not str:
-                raise TealInputError("Unknown type for base: {}".format(type(arg1)))
+                raise TealInputError(f"Unknown type for base: {type(arg1)}")
 
             if type(arg2) is not str:
-                raise TealInputError("Unknown type for value: {}".format(type(arg2)))
+                raise TealInputError(f"Unknown type for value: {type(arg2)}")
 
             self.base = arg1
 
@@ -75,9 +75,7 @@ def __init__(self, arg1: Union[str, bytes, bytearray], arg2: str = None) -> None
                 valid_base16(self.byte_str)
             else:
                 raise TealInputError(
-                    "invalid base {}, need to be base32, base64, or base16.".format(
-                        self.base
-                    )
+                    f"invalid base {self.base}, need to be base32, base64, or base16."
                 )
 
     def __teal__(self, options: "CompileOptions"):
@@ -86,12 +84,12 @@ def __teal__(self, options: "CompileOptions"):
         elif self.base == "base16":
             payload = "0x" + self.byte_str
         else:
-            payload = "{}({})".format(self.base, self.byte_str)
+            payload = f"{self.base}({self.byte_str})"
         op = TealOp(self, Op.byte, payload)
         return TealBlock.FromOp(options, op)
 
     def __str__(self):
-        return "({} bytes: {})".format(self.base, self.byte_str)
+        return f"({self.base} bytes: {self.byte_str})"
 
     def type_of(self):
         return TealType.bytes
diff --git a/pyteal/ast/bytes_test.py b/pyteal/ast/bytes_test.py
index 2ad5382a5..b14544737 100644
--- a/pyteal/ast/bytes_test.py
+++ b/pyteal/ast/bytes_test.py
@@ -1,6 +1,7 @@
 import pytest
 
 import pyteal as pt
+from pyteal.errors import TealInputError
 
 options = pt.CompileOptions()
 
@@ -88,6 +89,15 @@ def test_bytes_base16_empty():
     assert actual == expected
 
 
+B16_ODD_LEN_TESTCASES = ["F", "0c1"]
+
+
+@pytest.mark.parametrize("testcase", B16_ODD_LEN_TESTCASES)
+def test_bytes_base16_odd_len(testcase):
+    with pytest.raises(TealInputError):
+        pt.Bytes("base16", testcase)
+
+
 def test_bytes_utf8():
     expr = pt.Bytes("hello world")
     assert expr.type_of() == pt.TealType.bytes
diff --git a/pyteal/types.py b/pyteal/types.py
index 63b61bd54..a31ecbc0f 100644
--- a/pyteal/types.py
+++ b/pyteal/types.py
@@ -84,6 +84,11 @@ def valid_base64(s: str):
 
 def valid_base16(s: str):
     """check if s is a valid hex encoding string"""
+    if len(s) % 2:
+        raise TealInputError(
+            f"{s} is of odd length, not a valid RFC 4648 base16 string"
+        )
+
     pattern = re.compile(r"[0-9A-Fa-f]*")
 
     if pattern.fullmatch(s) is None: