diff --git a/.gitignore b/.gitignore
index 2c6ddfd..6559e83 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,4 @@ bundles
 dist
 **/*.egg-info
 .vscode
+.venv
diff --git a/adafruit_pioasm.py b/adafruit_pioasm.py
index 96fb8dd..07137fe 100644
--- a/adafruit_pioasm.py
+++ b/adafruit_pioasm.py
@@ -103,7 +103,12 @@ def assemble(text_program):
                 raise SyntaxError(f"Invalid jmp target {repr(target)}")
 
             if len(instruction) > 2:
-                assembled[-1] |= CONDITIONS.index(instruction[1]) << 5
+                try:
+                    assembled[-1] |= CONDITIONS.index(instruction[1]) << 5
+                except ValueError as exc:
+                    raise ValueError(
+                        f"Invalid jmp condition '{instruction[1]}'"
+                    ) from exc
 
         elif instruction[0] == "wait":
             #                instr delay p sr index
@@ -151,7 +156,10 @@ def assemble(text_program):
             source = instruction[-1]
             source_split = mov_splitter(source)
             if len(source_split) == 1:
-                assembled[-1] |= MOV_SOURCES.index(source)
+                try:
+                    assembled[-1] |= MOV_SOURCES.index(source)
+                except ValueError as exc:
+                    raise ValueError(f"Invalid mov source '{source}'") from exc
             else:
                 assembled[-1] |= MOV_SOURCES.index(source_split[1])
                 if source[:1] == "!":
@@ -183,7 +191,10 @@ def assemble(text_program):
         elif instruction[0] == "set":
             #                instr delay dst data
             assembled.append(0b111_00000_000_00000)
-            assembled[-1] |= SET_DESTINATIONS.index(instruction[1]) << 5
+            try:
+                assembled[-1] |= SET_DESTINATIONS.index(instruction[1]) << 5
+            except ValueError as exc:
+                raise ValueError(f"Invalid set destination '{instruction[1]}'") from exc
             value = int(instruction[-1])
             if not 0 <= value <= 31:
                 raise RuntimeError("Set value out of range")
diff --git a/tests/testpioasm.py b/tests/testpioasm.py
index a15d01c..3ef3be5 100644
--- a/tests/testpioasm.py
+++ b/tests/testpioasm.py
@@ -29,8 +29,11 @@ def assertAssemblesTo(self, source, expected):
             f"Assembling {source!r}: Expected {expected_bin}, got {actual_bin}",
         )
 
-    def assertAssemblyFails(self, source):
-        self.assertRaises(RuntimeError, adafruit_pioasm.assemble, source)
+    def assertAssemblyFails(self, source, match=None, errtype=RuntimeError):
+        if match:
+            self.assertRaisesRegex(errtype, match, adafruit_pioasm.assemble, source)
+        else:
+            self.assertRaises(errtype, adafruit_pioasm.assemble, source)
 
     def testNonsense(self):
         self.assertAssemblyFails("nope")
@@ -52,6 +55,18 @@ def testSidesetOpt(self):
         )
         self.assertAssemblesTo(".side_set 1 opt\nnop [1]", [0b101_00001_010_00_010])
 
+    def testMov(self):
+        # non happy path
+        self.assertAssemblyFails(
+            "mov x, blah", match="Invalid mov source 'blah'", errtype=ValueError
+        )
+
+    def testSet(self):
+        # non happy path
+        self.assertAssemblyFails(
+            "set isr, 1", match="Invalid set destination 'isr'", errtype=ValueError
+        )
+
     def testJmp(self):
         self.assertAssemblesTo("l:\njmp l", [0b000_00000_000_00000])
         self.assertAssemblesTo("l:\njmp 7", [0b000_00000_000_00111])
@@ -63,6 +78,10 @@ def testJmp(self):
         self.assertAssemblesTo("jmp x!=y, l\nl:", [0b000_00000_101_00001])
         self.assertAssemblesTo("jmp pin, l\nl:", [0b000_00000_110_00001])
         self.assertAssemblesTo("jmp !osre, l\nl:", [0b000_00000_111_00001])
+        # non happy path
+        self.assertAssemblyFails(
+            "jmp x--., l\nl:", match="Invalid jmp condition 'x--.'", errtype=ValueError
+        )
 
     def testWait(self):
         self.assertAssemblesTo("wait 0 gpio 0", [0b001_00000_0_00_00000])