diff --git a/aiida_hyperqueue/scheduler.py b/aiida_hyperqueue/scheduler.py
index 6c58005..e74412a 100644
--- a/aiida_hyperqueue/scheduler.py
+++ b/aiida_hyperqueue/scheduler.py
@@ -62,25 +62,26 @@ def validate_resources(cls, **kwargs):
         try:
             resources.num_cpus = kwargs.pop("num_cpus")
         except KeyError:
+            # For backward compatibility where `num_machines` and `num_mpiprocs_per_machine`
+            # are set. `num_mpiprocs_per_machine` is normally injected by aiida-core's
+            # `Scheduler.preprocess_resources` from the computer's
+            # `default_mpiprocs_per_machine`, which is `None` when the computer defines no
+            # default — hence the `or`-fallback to 1 rather than a `pop` default.
+            num_mpiprocs_per_machine = kwargs.pop("num_mpiprocs_per_machine", None) or 1
             try:
-                # For backward compatibility where `num_machines` and `num_mpiprocs_per_machine` are setting
-                # TODO: I only setting the default value as 1 for `num_mpiprocs_per_machine` because aiida-quantumespresso override
-                # resources default with `num_machines` set to 1 and then get builder with such setting.
-                # The `num_mpiprocs_per_machine` sometime can be read from "Default #procs/machine" of computer setup but if it is not exist
-                # the builder can not be properly get without passing `option` to builder generator.
-                # It is anyway a workaround for backward compatibility so this default is implemented despite it is quite specific for the qe plugin.
-                resources.num_cpus = kwargs.pop("num_machines") * kwargs.pop(
-                    "num_mpiprocs_per_machine", 1
+                resources.num_cpus = (
+                    kwargs.pop("num_machines") * num_mpiprocs_per_machine
                 )
             except KeyError:
                 raise KeyError(
                     "Must specify `num_cpus`, or (`num_machines` and `num_mpiprocs_per_machine`)"
                 )
             else:
-                message = "The `num_machines` and `num_mpiprocs_per_machine` for setting hyperqueue resources are deprecated. "
-                "Please set `num_cpus` and `memory_mb`."
-
-                message = f"{message} (this will be removed in aiida-hyperqueue v1.0)"
+                message = (
+                    "The `num_machines` and `num_mpiprocs_per_machine` for setting hyperqueue "
+                    "resources are deprecated. Please set `num_cpus` and `memory_mb`. "
+                    "(this will be removed in aiida-hyperqueue v1.0)"
+                )
                 warnings.warn(message, AiiDAHypereQueueDeprecationWarning, stacklevel=3)
         else:
             if not isinstance(resources.num_cpus, int):
@@ -98,8 +99,13 @@ def validate_resources(cls, **kwargs):
 
     @classmethod
     def accepts_default_mpiprocs_per_machine(cls):
-        """Return True if this subclass accepts a `default_mpiprocs_per_machine` key, False otherwise."""
-        return False
+        """Return True if this subclass accepts a `default_mpiprocs_per_machine` key, False otherwise.
+
+        Accepting it lets the backward-compatibility `num_machines` path pick up the
+        computer's ``default_mpiprocs_per_machine`` instead of silently running on a
+        single CPU.
+        """
+        return True
 
     def get_tot_num_mpiprocs(self):
         """Return the total number of cpus of this job resource."""
diff --git a/tests/test_scheduler.py b/tests/test_scheduler.py
index b5c47c2..fac1c5c 100644
--- a/tests/test_scheduler.py
+++ b/tests/test_scheduler.py
@@ -65,6 +65,37 @@ def test_resource_validation():
         HyperQueueJobResource(num_cpus=4, memory_mb=1.2)
 
 
+def test_resource_validation_backward_compatibility():
+    """Tests for the deprecated `num_machines` / `num_mpiprocs_per_machine` path."""
+    # num_cpus is the product of the two legacy keys
+    with pytest.warns(Warning, match="deprecated"):
+        resource = HyperQueueJobResource(num_machines=2, num_mpiprocs_per_machine=8)
+    assert resource.num_cpus == 16
+
+    # the computer's default_mpiprocs_per_machine, which aiida-core's
+    # `Scheduler.preprocess_resources` injects as `num_mpiprocs_per_machine`,
+    # must be honoured (this is what `accepts_default_mpiprocs_per_machine`
+    # returning True enables)
+    with pytest.warns(Warning, match="deprecated"):
+        resource = HyperQueueJobResource(num_machines=1, num_mpiprocs_per_machine=8)
+    assert resource.num_cpus == 8
+
+    # a computer without a default leads aiida-core to inject `None`; fall back to 1
+    with pytest.warns(Warning, match="deprecated"):
+        resource = HyperQueueJobResource(num_machines=1, num_mpiprocs_per_machine=None)
+    assert resource.num_cpus == 1
+
+    # `num_mpiprocs_per_machine` omitted entirely also falls back to 1
+    with pytest.warns(Warning, match="deprecated"):
+        resource = HyperQueueJobResource(num_machines=1)
+    assert resource.num_cpus == 1
+
+
+def test_accepts_default_mpiprocs_per_machine():
+    """The resource class must accept the computer's default so it reaches validate_resources."""
+    assert HyperQueueJobResource.accepts_default_mpiprocs_per_machine()
+
+
 def test_submit_command():
     """Test submit command"""
     scheduler = HyperQueueScheduler()