diff --git a/src/ion/liveranges.rs b/src/ion/liveranges.rs
index fefff2f..062917c 100644
--- a/src/ion/liveranges.rs
+++ b/src/ion/liveranges.rs
@@ -368,19 +368,15 @@ impl<'a, F: Function> Env<'a, F> {
             self.liveins[block.index()] = live;
         }
 
-        // Check that there are no liveins to the entry block. (The
-        // client should create a virtual intsruction that defines any
-        // PReg liveins if necessary.)
-        if self.liveins[self.func.entry_block().index()]
-            .iter()
-            .next()
-            .is_some()
-        {
-            trace!(
-                "non-empty liveins to entry block: {:?}",
-                self.liveins[self.func.entry_block().index()]
-            );
-            return Err(RegAllocError::EntryLivein);
+        // Check that there are no liveins to the entry block, except
+        // for pinned vregs. (The client should create a virtual
+        // instruction that defines any other liveins if necessary.)
+        for livein in self.liveins[self.func.entry_block().index()].iter() {
+            let livein = self.vreg_regs[livein];
+            if self.func.is_pinned_vreg(livein).is_none() {
+                trace!("non-pinned-vreg livein to entry block: {}", livein);
+                return Err(RegAllocError::EntryLivein);
+            }
         }
 
         Ok(())
diff --git a/src/lib.rs b/src/lib.rs
index 73a6c1c..29521ae 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -977,6 +977,11 @@ pub trait Function {
     /// liverange computation will check that this is the case (that
     /// there are enough remaining allocatable pregs of every class to
     /// hold all Reg-constrained operands).
+    ///
+    /// Pinned vregs are implicitly live-in to the function: that is,
+    /// one can use a pinned vreg without having first defined it, and
+    /// this will take the value that that physical register (to which
+    /// the vreg is pinned) had at function entry.
     fn is_pinned_vreg(&self, _: VReg) -> Option<PReg> {
         None
     }