diff --git a/qualtran/_infra/composite_bloq.py b/qualtran/_infra/composite_bloq.py
index ee9448600..456be5598 100644
--- a/qualtran/_infra/composite_bloq.py
+++ b/qualtran/_infra/composite_bloq.py
@@ -846,6 +846,8 @@ def add_register_from_dtype(
             initial, left-dangling soquets for the register. Otherwise, this is a RIGHT register
             and will be used for error checking in `finalize()` and nothing is returned.
         """
+        from qualtran.symbolics import is_symbolic
+
         if not self.add_register_allowed:
             raise ValueError(
                 "This BloqBuilder was constructed from pre-specified registers. "
@@ -864,6 +866,11 @@ def add_register_from_dtype(
                 )
             reg = Register(name=reg, dtype=dtype)
 
+        if is_symbolic(*reg.shape_symbolic):
+            raise DecomposeTypeError(
+                f"cannot add register with symbolic shape {reg.shape_symbolic}"
+            )
+
         self._regs.append(reg)
         if reg.side & Side.LEFT:
             return _reg_to_soq(LeftDangle, reg, available=self._available)
diff --git a/qualtran/_infra/composite_bloq_test.py b/qualtran/_infra/composite_bloq_test.py
index 5685641ab..34d42e991 100644
--- a/qualtran/_infra/composite_bloq_test.py
+++ b/qualtran/_infra/composite_bloq_test.py
@@ -20,6 +20,7 @@
 import networkx as nx
 import numpy as np
 import pytest
+import sympy
 from numpy.typing import NDArray
 
 import qualtran.testing as qlt_testing
@@ -30,6 +31,7 @@
     BloqInstance,
     CompositeBloq,
     Connection,
+    DecomposeTypeError,
     LeftDangle,
     Register,
     RightDangle,
@@ -45,6 +47,7 @@
 from qualtran.bloqs.for_testing.atom import TestAtom, TestTwoBitOp
 from qualtran.bloqs.for_testing.many_registers import TestMultiTypedRegister, TestQFxp
 from qualtran.bloqs.for_testing.with_decomposition import TestParallelCombo, TestSerialCombo
+from qualtran.symbolics import SymbolicInt
 
 
 def _manually_make_test_cbloq_cxns():
@@ -597,6 +600,22 @@ def test_add_and_partition():
     assert len(cbloq.bloq_instances) == 1
 
 
+@attrs.frozen
+class TestSymbolicRegisterShape(Bloq):
+    n: 'SymbolicInt'
+
+    @property
+    def signature(self) -> 'Signature':
+        return Signature([Register('q', QBit(), shape=(self.n,))])
+
+
+def test_decompose_symbolic_register_shape_raises():
+    n = sympy.Symbol("n")
+    bloq = TestSymbolicRegisterShape(n)
+    with pytest.raises(DecomposeTypeError):
+        bloq.decompose_bloq()
+
+
 @pytest.mark.notebook
 def test_notebook():
     qlt_testing.execute_notebook('composite_bloq')
diff --git a/qualtran/cirq_interop/_cirq_to_bloq.py b/qualtran/cirq_interop/_cirq_to_bloq.py
index 078051bb1..3911b3e24 100644
--- a/qualtran/cirq_interop/_cirq_to_bloq.py
+++ b/qualtran/cirq_interop/_cirq_to_bloq.py
@@ -585,7 +585,7 @@ def decompose_from_registers(
             yields the cirq-style decomposition.
     """
     if any(
-        cirq.is_parameterized(reg.bitsize) or cirq.is_parameterized(reg.side)
+        cirq.is_parameterized(reg.bitsize) or cirq.is_parameterized(reg.side) or reg.is_symbolic()
         for reg in bloq.signature
     ):
         # pylint: disable=raise-missing-from