diff --git a/src/back/spv/writer.rs b/src/back/spv/writer.rs
index 189876b003..02485c02a7 100644
--- a/src/back/spv/writer.rs
+++ b/src/back/spv/writer.rs
@@ -1555,10 +1555,6 @@ impl Writer {
                 kind,
                 convert,
             } => {
-                if !convert {
-                    return Err(Error::FeatureNotImplemented("bitcast"));
-                }
-
                 let expr_id =
                     self.write_expression(ir_module, ir_function, expr, block, function)?;
                 let expr_kind = self
diff --git a/src/front/spv/mod.rs b/src/front/spv/mod.rs
index 16ab05d80c..2bf98b8b91 100644
--- a/src/front/spv/mod.rs
+++ b/src/front/spv/mod.rs
@@ -1466,9 +1466,29 @@ impl<I: Iterator<Item = u32>> Parser<I> {
                     inst.expect_at_least(3)?;
 
                     let selector = self.next()?;
-                    let selector = self.lookup_expression[&selector].handle;
                     let default = self.next()?;
 
+                    let selector_lexp = &self.lookup_expression[&selector];
+                    let selector_lty = self.lookup_type.lookup(selector_lexp.type_id)?;
+                    let selector = match type_arena[selector_lty.handle].inner {
+                        crate::TypeInner::Scalar {
+                            kind: crate::ScalarKind::Uint,
+                            width: _,
+                        } => {
+                            // IR expects a signed integer, so do a bitcast
+                            expressions.append(crate::Expression::As {
+                                kind: crate::ScalarKind::Sint,
+                                expr: selector_lexp.handle,
+                                convert: false,
+                            })
+                        }
+                        crate::TypeInner::Scalar {
+                            kind: crate::ScalarKind::Sint,
+                            width: _,
+                        } => selector_lexp.handle,
+                        ref other => unimplemented!("Unexpected selector {:?}", other),
+                    };
+
                     let mut targets = Vec::new();
                     for _ in 0..(inst.wc - 3) / 2 {
                         let literal = self.next()?;