diff --git a/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c b/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
index 65991aff0254..8b5ce9f46726 100644
--- a/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
+++ b/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
@@ -272,7 +272,20 @@ fasttrap_anarg(struct reg *rp, int function_entry, int argno)
 		 * registers.
 		 */
 		if (argno < 6)
-			return ((&rp->r_rdi)[argno]);
+			switch (argno) {
+			case 0:
+				return (rp->r_rdi);
+			case 1:
+				return (rp->r_rsi);
+			case 2:
+				return (rp->r_rdx);
+			case 3:
+				return (rp->r_rcx);
+			case 4:
+				return (rp->r_r8);
+			case 5:
+				return (rp->r_r9);
+			}
 
 		stack = (uintptr_t *)rp->r_rsp;
 		DTRACE_CPUFLAG_SET(CPU_DTRACE_NOFAULT);
diff --git a/sys/cddl/dev/dtrace/amd64/dtrace_isa.c b/sys/cddl/dev/dtrace/amd64/dtrace_isa.c
index 34d6f331ebee..3d3c43cd2e8a 100644
--- a/sys/cddl/dev/dtrace/amd64/dtrace_isa.c
+++ b/sys/cddl/dev/dtrace/amd64/dtrace_isa.c
@@ -367,7 +367,27 @@ dtrace_getarg(int arg, int aframes)
 			    sizeof (uintptr_t));
 
 			if (arg <= inreg) {
-				stack = (uintptr_t *)&rp->r_rdi;
+				switch (arg) {
+				case 0:
+					stack = (uintptr_t *)&rp->r_rdi;
+					break;
+				case 1:
+					stack = (uintptr_t *)&rp->r_rsi;
+					break;
+				case 2:
+					stack = (uintptr_t *)&rp->r_rdx;
+					break;
+				case 3:
+					stack = (uintptr_t *)&rp->r_rcx;
+					break;
+				case 4:
+					stack = (uintptr_t *)&rp->r_r8;
+					break;
+				case 5:
+					stack = (uintptr_t *)&rp->r_r9;
+					break;
+				}
+				arg = 0;
 			} else {
 				stack = (uintptr_t *)(rp->r_rsp);
 				arg -= inreg;