diff --git a/src/single_cell/ApPredictMethods.cpp b/src/single_cell/ApPredictMethods.cpp
index 33ef7d6..e80a13d 100644
--- a/src/single_cell/ApPredictMethods.cpp
+++ b/src/single_cell/ApPredictMethods.cpp
@@ -250,6 +250,20 @@ void ApPredictMethods::ReadInIC50HillAndSaturation(
         read_ic50s = true;
     }
 
+    // If the user has requested a block on this channel, but the model doesn't
+    // have this conductance labelled, throw straight away. Otherwise this
+    // wouldn't be picked up until we reach a concentration in the main loop
+    // that actually causes some block (i.e. a non-zero concentration), which
+    // can be confusing as the simulation appears to run fine for the first
+    // (often zero, "control") concentration(s) before failing later on.
+    if (read_ic50s && !mpModel->HasParameter(mMetadataNames[channelIdx]) && !mpModel->HasParameter(mMetadataNames[channelIdx] + "_scaling_factor"))
+    {
+        EXCEPTION(
+            mpModel->GetSystemName()
+            << " does not have the current \"" << mMetadataNames[channelIdx]
+            << "\" labelled, but you have requested a block on this channel.");
+    }
+
     // Try loading any Hills
     if (p_args->OptionExists("--hill-" + channel))
     {
@@ -399,16 +413,10 @@ void ApPredictMethods::ApplyDrugBlock(
     }
     else // We haven't got that conductance parameter, or at least it isn't labelled.
     {
-        // If we aren't trying to change it - don't worry, just carry on.
-        if (conductance_factor < 1)
-        {
-            // If the model hasn't got this channel conductance labelled,
-            // (but we are trying to change it) throw an error.
-            EXCEPTION(
-                pModel->GetSystemName()
-                << " does not have the current \"" << mMetadataNames[channel_index]
-                << "\" labelled, but you have requested a block on this channel.");
-        }
+        // We aren't trying to change it (ReadInIC50HillAndSaturation() already throws,
+        // before we ever get here, if a block was requested on a channel that isn't
+        // labelled in this model) - don't worry, just carry on.
+        assert(conductance_factor == 1.0);
     }
 }
 
diff --git a/test/TestApPredict.hpp b/test/TestApPredict.hpp
index a4ad694..0fcff00 100644
--- a/test/TestApPredict.hpp
+++ b/test/TestApPredict.hpp
@@ -123,6 +123,21 @@ class TestApPredict : public CxxTest::TestSuite
             TS_ASSERT_THROWS_THIS(SetupModel setup(1.0, UNSIGNED_UNSET),
                                   "Invalid file given with --cellml argument: bla.cellml");
         }
+        {
+            // See https://github.com/Chaste/ApPredict/issues/3 - requesting a block on a
+            // channel that isn't labelled in the model should be picked up immediately,
+            // rather than only once we reach a concentration in the loop that actually
+            // causes some conductance change (which was confusing, as it could look like
+            // the simulation had run successfully for the earlier, e.g. zero, concentrations).
+            CommandLineArgumentsMocker wrapper("--model 1 --pacing-freq 1.0 --pacing-max-time 5 "
+                                                "--pic50-nal 1 --hill-nal 1 --saturation-nal 0 "
+                                                "--plasma-conc-high 100 --plasma-conc-low 0 "
+                                                "--plasma-conc-count 4 --plasma-conc-logscale true");
+
+            ApPredictMethods methods;
+            TS_ASSERT_THROWS_CONTAINS(methods.Run(),
+                                      "does not have the current \"membrane_persistent_sodium_current_conductance\" labelled, but you have requested a block on this channel.");
+        }
     }
 
     void TestVoltageThresholdDetectionAlgorithm()