diff --git a/qiskit/aqua/operators/converters/circuit_sampler.py b/qiskit/aqua/operators/converters/circuit_sampler.py
index c4190554c8d..1000a128da0 100644
--- a/qiskit/aqua/operators/converters/circuit_sampler.py
+++ b/qiskit/aqua/operators/converters/circuit_sampler.py
@@ -21,7 +21,7 @@
 from qiskit.providers import Backend
 from qiskit.circuit import QuantumCircuit, Parameter, ParameterExpression
 from qiskit import QiskitError
-from qiskit.aqua import QuantumInstance
+from qiskit.aqua import QuantumInstance, AquaError
 from qiskit.aqua.utils.backend_utils import is_aer_provider, is_statevector_backend
 from qiskit.aqua.operators.operator_base import OperatorBase
 from qiskit.aqua.operators.list_ops.list_op import ListOp
@@ -165,6 +165,8 @@ def convert(self,
 
         Returns:
             The converted Operator with CircuitStateFns replaced by DictStateFns or VectorStateFns.
+        Raises:
+            AquaError: if extracted circuits are empty.
         """
         if self._last_op is None or id(operator) != id(self._last_op):
             # Clear caches
@@ -181,6 +183,11 @@ def convert(self,
         if not self._circuit_ops_cache:
             self._circuit_ops_cache = {}
             self._extract_circuitstatefns(self._reduced_op_cache)
+            if not self._circuit_ops_cache:
+                raise AquaError(
+                    'Circuits are empty. '
+                    'Check that the operator is an instance of CircuitStateFn or its ListOp.'
+                )
 
         if params:
             p_0 = list(params.values())[0]  # type: ignore
@@ -243,8 +250,13 @@ def sample_circuits(self,
 
         Returns:
             The dictionary mapping ids of the CircuitStateFns to their replacement StateFns.
+        Raises:
+            AquaError: if extracted circuits are empty.
         """
-        if circuit_sfns or not self._transpiled_circ_cache:
+        if not circuit_sfns and not self._transpiled_circ_cache:
+            raise AquaError('CircuitStateFn is empty and there is no cache.')
+
+        if circuit_sfns:
             if self._statevector:
                 circuits = [op_c.to_circuit(meas=False) for op_c in circuit_sfns]
             else:
diff --git a/qiskit/aqua/operators/evolutions/trotterizations/trotter.py b/qiskit/aqua/operators/evolutions/trotterizations/trotter.py
index 3152c1d1a5f..e2ed34768ed 100644
--- a/qiskit/aqua/operators/evolutions/trotterizations/trotter.py
+++ b/qiskit/aqua/operators/evolutions/trotterizations/trotter.py
@@ -26,4 +26,4 @@ def __init__(self,
         Args:
             reps: The number of times to repeat the Trotterization circuit.
         """
-        super().__init__(order=1, reps=1)
+        super().__init__(order=1, reps=reps)
diff --git a/qiskit/aqua/operators/legacy/weighted_pauli_operator.py b/qiskit/aqua/operators/legacy/weighted_pauli_operator.py
index 4f1ab276491..8192284b3ed 100644
--- a/qiskit/aqua/operators/legacy/weighted_pauli_operator.py
+++ b/qiskit/aqua/operators/legacy/weighted_pauli_operator.py
@@ -101,9 +101,19 @@ def to_opflow(self, reverse_endianness=False):
         pauli_ops = []
         for [w, p] in self.paulis:
             pauli = Pauli.from_label(str(p)[::-1]) if reverse_endianness else p
-            # Adding the imaginary is necessary to handle the imaginary coefficients in UCCSD.
-            # TODO fix these or add support for them in Terra.
-            pauli_ops += [PrimitiveOp(pauli, coeff=np.real(w) + np.imag(w))]
+            # This weighted pauli operator has the coeff stored as a complex type
+            # irrespective of whether the value has any imaginary part or not.
+            # For many operators the coeff will be real. Hence below the coeff is made real,
+            # when creating the PrimitiveOp, since it can be stored then as a float, if its
+            # value is real, i.e. has no imaginary part. This avoids any potential issues around
+            # complex - but if there are complex coeffs then maybe that using the opflow
+            # later will fail if it happens to be used where complex is not supported.
+            # Now there are imaginary coefficients in UCCSD that would need to be handled
+            # when this is converted to opflow (evolution of hopping operators) where currently
+            # Terra does not handle complex.
+            # TODO fix these or add support for them in Terra
+            coeff = np.real(w) if np.isreal(w) else w
+            pauli_ops += [PrimitiveOp(pauli, coeff=coeff)]
         return sum(pauli_ops)
 
     @property
diff --git a/qiskit/aqua/operators/state_fns/dict_state_fn.py b/qiskit/aqua/operators/state_fns/dict_state_fn.py
index 8127a5aaf22..734aea8f5c7 100644
--- a/qiskit/aqua/operators/state_fns/dict_state_fn.py
+++ b/qiskit/aqua/operators/state_fns/dict_state_fn.py
@@ -252,7 +252,7 @@ def sample(self,
                shots: int = 1024,
                massive: bool = False,
                reverse_endianness: bool = False) -> dict:
-        probs = np.array(list(self.primitive.values()))**2
+        probs = np.square(np.abs(np.array(list(self.primitive.values()))))
         unique, counts = np.unique(aqua_globals.random.choice(list(self.primitive.keys()),
                                                               size=shots,
                                                               p=(probs / sum(probs))),
diff --git a/qiskit/aqua/operators/state_fns/operator_state_fn.py b/qiskit/aqua/operators/state_fns/operator_state_fn.py
index d7476a3b4ba..97adbac61ea 100644
--- a/qiskit/aqua/operators/state_fns/operator_state_fn.py
+++ b/qiskit/aqua/operators/state_fns/operator_state_fn.py
@@ -34,7 +34,7 @@ class OperatorStateFn(StateFn):
 
     # TODO allow normalization somehow?
     def __init__(self,
-                 primitive: Union[OperatorBase] = None,
+                 primitive: OperatorBase = None,
                  coeff: Union[int, float, complex, ParameterExpression] = 1.0,
                  is_measurement: bool = False) -> None:
         """